diff --git a/src/main/java/cc/sukazyo/messiva/formatter/SimpleFormatter.java b/src/main/java/cc/sukazyo/messiva/formatter/SimpleFormatter.java
index 60d7ee2..43c7c7b 100644
--- a/src/main/java/cc/sukazyo/messiva/formatter/SimpleFormatter.java
+++ b/src/main/java/cc/sukazyo/messiva/formatter/SimpleFormatter.java
@@ -14,6 +14,7 @@ public class SimpleFormatter implements ILogFormatter {
 	@Nonnull public String startLevelTag = "[";
 	@Nonnull public String endLevelTag = "]";
 	public char followingLineFillChar = '\'';
+	@Nonnull public String messageSeparator = " ";
 	
 	@Nonnull
 	@Override
@@ -22,10 +23,12 @@ public class SimpleFormatter implements ILogFormatter {
 		message.append(startTimestamp).append(log.timestamp).append(endTimestamp)
 				.append(startThreadName).append(log.thread.getName()).append(endThreadName);
 		final String promptNewline = StringUtils.repeatChar(followingLineFillChar, message.length());
-		message.append(startLevelTag).append(log.level.tag()).append(endLevelTag).append(log.message.message[0]);
+		message.append(startLevelTag).append(log.level.tag()).append(endLevelTag)
+				.append(messageSeparator).append(log.message.message[0]);
 		for (int i = 1; i < log.message.message.length; i++) {
 			message.append('\n').append(promptNewline)
-					.append(startLevelTag).append(log.level.tag()).append(endLevelTag).append(log.message.message[i]);
+					.append(startLevelTag).append(log.level.tag()).append(endLevelTag)
+					.append(messageSeparator).append(log.message.message[i]);
 		}
 		return message.toString();
 	}
diff --git a/src/main/java/cc/sukazyo/messiva/log/ILogLevel.java b/src/main/java/cc/sukazyo/messiva/log/ILogLevel.java
index 6f3caa5..cd0c571 100644
--- a/src/main/java/cc/sukazyo/messiva/log/ILogLevel.java
+++ b/src/main/java/cc/sukazyo/messiva/log/ILogLevel.java
@@ -1,6 +1,8 @@
 package cc.sukazyo.messiva.log;
 
+import javax.annotation.Nonnull;
+
 public interface ILogLevel {
 	float level();
-	String tag();
+	@Nonnull String tag();
 }
diff --git a/src/main/java/cc/sukazyo/messiva/log/ILogLevelImpl.java b/src/main/java/cc/sukazyo/messiva/log/ILogLevelImpl.java
index b2d657c..b07f5e4 100644
--- a/src/main/java/cc/sukazyo/messiva/log/ILogLevelImpl.java
+++ b/src/main/java/cc/sukazyo/messiva/log/ILogLevelImpl.java
@@ -1,33 +1,35 @@
 package cc.sukazyo.messiva.log;
 
+import javax.annotation.Nonnull;
+
 public interface ILogLevelImpl {
 	
-	void trace (String message);
+	void trace (@Nonnull String message);
 	
-	void debug (String message);
+	void debug (@Nonnull String message);
 	
-	void info (String message);
+	void info (@Nonnull String message);
 	
-	void warn (String message);
+	void warn (@Nonnull String message);
 	
-	void warning (String message);
+	void warning (@Nonnull String message);
 	
-	void error (String message);
+	void error (@Nonnull String message);
 	
-	void fatal (String message);
+	void fatal (@Nonnull String message);
 	
-	void trace (Message message);
+	void trace (@Nonnull Message message);
 	
-	void debug (Message message);
+	void debug (@Nonnull Message message);
 	
-	void info (Message message);
+	void info (@Nonnull Message message);
 	
-	void warn (Message message);
+	void warn (@Nonnull Message message);
 	
-	void warning (Message message);
+	void warning (@Nonnull Message message);
 	
-	void error (Message message);
+	void error (@Nonnull Message message);
 	
-	void fatal (Message message);
+	void fatal (@Nonnull Message message);
 	
 }
diff --git a/src/main/java/cc/sukazyo/messiva/log/LogLevels.java b/src/main/java/cc/sukazyo/messiva/log/LogLevels.java
index 8d3284d..3096722 100644
--- a/src/main/java/cc/sukazyo/messiva/log/LogLevels.java
+++ b/src/main/java/cc/sukazyo/messiva/log/LogLevels.java
@@ -1,5 +1,7 @@
 package cc.sukazyo.messiva.log;
 
+import javax.annotation.Nonnull;
+
 public enum LogLevels implements ILogLevel {
 	
 	ALL  (-Float.MAX_VALUE, "####"),
@@ -12,9 +14,9 @@ public enum LogLevels implements ILogLevel {
 	NONE(Float.MAX_VALUE,  "!!!!");
 	
 	public final float level;
-	public final String tag;
+	@Nonnull public final String tag;
 	
-	LogLevels (float level, String tag) {
+	LogLevels (float level, @Nonnull String tag) {
 		this.level = level;
 		this.tag = tag;
 	}
@@ -22,6 +24,7 @@ public enum LogLevels implements ILogLevel {
 	@Override
 	public float level () { return this.level; }
 	
+	@Nonnull
 	@Override
 	public String tag () { return this.tag; }
 	
diff --git a/src/main/java/cc/sukazyo/messiva/log/Message.java b/src/main/java/cc/sukazyo/messiva/log/Message.java
index c51b725..50dd88b 100644
--- a/src/main/java/cc/sukazyo/messiva/log/Message.java
+++ b/src/main/java/cc/sukazyo/messiva/log/Message.java
@@ -1,10 +1,14 @@
 package cc.sukazyo.messiva.log;
 
+import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
+
 public class Message {
 	
+	@Nonnull
 	public final String[] message;
 	
-	public Message(String message) {
+	public Message(@Nullable String message) {
 		this.message = message == null ? new String[]{} : message.split("\n");
 	}
 	
diff --git a/src/main/java/cc/sukazyo/messiva/logger/Logger.java b/src/main/java/cc/sukazyo/messiva/logger/Logger.java
index 194b3d9..a613106 100644
--- a/src/main/java/cc/sukazyo/messiva/logger/Logger.java
+++ b/src/main/java/cc/sukazyo/messiva/logger/Logger.java
@@ -20,82 +20,84 @@ public class Logger implements ILogLevelImpl {
 		appends = new ArrayList<>();
 	}
 	
-	public Logger (IAppender... appends) {
+	public Logger (@Nonnull IAppender... appends) {
 		this();
 		this.appends.addAll(Arrays.asList(appends));
 	}
 	
+	@Nonnull
 	public Logger minLevel (@Nonnull ILogLevel minLevel) {
 		levelSetting.minLevel(minLevel);
 		return this;
 	}
 	
+	@Nonnull
 	public Logger maxLevel (@Nonnull ILogLevel maxLevel) {
 		levelSetting.maxLevel(maxLevel);
 		return this;
 	}
 	
-	public void trace (String message) {
+	public void trace (@Nonnull String message) {
 		pushToAllAppender(new Log(1, new Message(message), LogLevels.TRACE));
 	}
 	
-	public void debug (String message) {
+	public void debug (@Nonnull String message) {
 		pushToAllAppender(new Log(1, new Message(message), LogLevels.DEBUG));
 	}
 	
-	public void info (String message) {
+	public void info (@Nonnull String message) {
 		pushToAllAppender(new Log(1, new Message(message), LogLevels.INFO));
 	}
 	
-	public void warn (String message) {
+	public void warn (@Nonnull String message) {
 		pushToAllAppender(new Log(1, new Message(message), LogLevels.WARN));
 	}
 	
-	public void warning (String message) {
+	public void warning (@Nonnull String message) {
 		pushToAllAppender(new Log(1, new Message(message), LogLevels.WARN));
 	}
 	
-	public void error (String message) {
+	public void error (@Nonnull String message) {
 		pushToAllAppender(new Log(1, new Message(message), LogLevels.ERROR));
 	}
 	
-	public void fatal (String message) {
+	public void fatal (@Nonnull String message) {
 		pushToAllAppender(new Log(1, new Message(message), LogLevels.FATAL));
 	}
 	
-	public void trace (Message message) {
+	public void trace (@Nonnull Message message) {
 		pushToAllAppender(new Log(1, message, LogLevels.TRACE));
 	}
 	
-	public void debug (Message message) {
+	public void debug (@Nonnull Message message) {
 		pushToAllAppender(new Log(1, message, LogLevels.DEBUG));
 	}
 	
-	public void info (Message message) {
+	public void info (@Nonnull Message message) {
 		pushToAllAppender(new Log(1, message, LogLevels.INFO));
 	}
 	
-	public void warn (Message message) {
+	public void warn (@Nonnull Message message) {
 		pushToAllAppender(new Log(1, message, LogLevels.WARN));
 	}
 	
-	public void warning (Message message) {
+	public void warning (@Nonnull Message message) {
 		pushToAllAppender(new Log(1, message, LogLevels.WARN));
 	}
 	
-	public void error (Message message) {
+	public void error (@Nonnull Message message) {
 		pushToAllAppender(new Log(1, message, LogLevels.ERROR));
 	}
 	
-	public void fatal (Message message) {
+	public void fatal (@Nonnull Message message) {
 		pushToAllAppender(new Log(1, message, LogLevels.FATAL));
 	}
 	
-	public void log (String message, ILogLevel level) {
+	public void log (@Nonnull String message, @Nonnull ILogLevel level) {
 		pushToAllAppender(new Log(1, new Message(message), level));
 	}
 	
-	public void log (Message message, ILogLevel level) {
+	public void log (@Nonnull Message message, @Nonnull ILogLevel level) {
 		pushToAllAppender(new Log(1, message, level));
 	}