diff --git a/libcobj/app/src/main/java/jp/osscons/opensourcecobol/libcobj/data/AbstractCobolField.java b/libcobj/app/src/main/java/jp/osscons/opensourcecobol/libcobj/data/AbstractCobolField.java
index 8a2fac8c..f72700b7 100644
--- a/libcobj/app/src/main/java/jp/osscons/opensourcecobol/libcobj/data/AbstractCobolField.java
+++ b/libcobj/app/src/main/java/jp/osscons/opensourcecobol/libcobj/data/AbstractCobolField.java
@@ -42,7 +42,13 @@ public abstract class AbstractCobolField {
     static CobolDataStorage lastdata = null;
 
     /** 文字データの符号化に使用するShift_JIS文字セット */
-    public static Charset charSetSJIS = Charset.forName("SHIFT-JIS");
+    /**
+     * Default charset for String/byte[] conversions.
+     * Configurable via COB_CHARSET environment variable.
+     * Defaults to SHIFT-JIS for backward compatibility.
+     */
+    public static Charset charSetSJIS =
+            Charset.forName(System.getenv().getOrDefault("COB_CHARSET", "SHIFT-JIS"));
 
     static final int[] cobExp10 = {
         1, 10, 100, 1000, 10000, 100000, 1000000, 10000000, 100000000, 1000000000