diff --git a/src/main/java/org/codelibs/fess/helper/DocumentHelper.java b/src/main/java/org/codelibs/fess/helper/DocumentHelper.java
index 40832cfe1..f2f1a95f4 100644
--- a/src/main/java/org/codelibs/fess/helper/DocumentHelper.java
+++ b/src/main/java/org/codelibs/fess/helper/DocumentHelper.java
@@ -252,4 +252,16 @@ public class DocumentHelper {
         return hash;
     }
 
+    public String appendLineNumber(final String prefix, final String content) {
+        if (content == null) {
+            return StringUtil.EMPTY;
+        }
+        final String[] values = content.split("\n");
+        final StringBuilder buf = new StringBuilder((int) (content.length() * 1.3));
+        buf.append(prefix).append(1).append(':').append(values[0]);
+        for (int i = 1; i < values.length; i++) {
+            buf.append('\n').append(prefix).append(i + 1).append(':').append(values[i]);
+        }
+        return buf.toString();
+    }
 }
diff --git a/src/test/java/org/codelibs/fess/helper/DocumentHelperTest.java b/src/test/java/org/codelibs/fess/helper/DocumentHelperTest.java
index 6d3c0bb53..544061aef 100644
--- a/src/test/java/org/codelibs/fess/helper/DocumentHelperTest.java
+++ b/src/test/java/org/codelibs/fess/helper/DocumentHelperTest.java
@@ -174,4 +174,15 @@ public class DocumentHelperTest extends UnitFessTestCase {
 
         assertNull(documentHelper.decodeSimilarDocHash(null));
     }
+
+    public void test_appendLineNumber() {
+        DocumentHelper documentHelper = new DocumentHelper();
+
+        assertEquals("", documentHelper.appendLineNumber("L", null));
+        assertEquals("L1:", documentHelper.appendLineNumber("L", ""));
+        assertEquals("L1:aaa", documentHelper.appendLineNumber("L", "aaa"));
+        assertEquals("L1:aaa", documentHelper.appendLineNumber("L", "aaa\n"));
+        assertEquals("L1:aaa\nL2:bbb", documentHelper.appendLineNumber("L", "aaa\nbbb"));
+        assertEquals("L1:aaa\nL2:bbb\nL3:ccc", documentHelper.appendLineNumber("L", "aaa\nbbb\nccc"));
+    }
 }