diff --git a/SMILEBASIC/TEST.txt b/SMILEBASIC/TEST.txt
index c031f3f..6fa6adf 100644
--- a/SMILEBASIC/TEST.txt
+++ b/SMILEBASIC/TEST.txt
@@ -32,6 +32,7 @@ ASSERT__ VAL("aa")==0, "VAL"
 
 ASSERT__ RAD(180)==PI(), "RAD"
 ASSERT__ DEG(RAD(180)), "DEG"
+ASSERT__ HEX$(65535, 8)=="0000FFFF", "HEX$"
 ASSERT__ SUBST$("0123456789",0,"B")=="B","SUBST$"
 ASSERT__ SUBST$("0123456789",0,4,"B")=="B456789","SUBST$"
 ASSERT__ SUBST$("",0,4,"B")=="B","SUBST$"
diff --git a/SMILEBASIC/builtinfunctions.d b/SMILEBASIC/builtinfunctions.d
index 20add47..66e21b2 100644
--- a/SMILEBASIC/builtinfunctions.d
+++ b/SMILEBASIC/builtinfunctions.d
@@ -365,10 +365,25 @@ class BuiltinFunction
     {
         return val.to!wstring;
     }
-    static wstring HEX(int val)
+    static wstring HEX(int val, DefaultValue!(int, false) digits)
     {
-        //ひどい
-        return val.to!string(16).to!wstring;
+        import std.format;
+        if(digits.isDefault)
+        {
+            //ひどい
+            return val.to!string(16).to!wstring;
+        }
+        if(digits > 8)
+        {
+            throw new OutOfRange();
+        }
+        FormatSpec!char f;
+        f.spec = 'X';
+        f.flZero = true;
+        f.width = cast(int)digits;
+        auto w = appender!wstring();
+        formatValue(w, val, f);
+        return cast(immutable)(w.data);
     }
     static void SPSET(PetitComputer p, int id, int defno)
     {
diff --git a/SMILEBASIC/error.d b/SMILEBASIC/error.d
index 9a3c8a2..3450d74 100644
--- a/SMILEBASIC/error.d
+++ b/SMILEBASIC/error.d
@@ -64,6 +64,14 @@ class TypeMismatch : SmileBasicError
         super("Type mismatch");
     }
 }
+class OutOfRange : SmileBasicError
+{
+    this()
+    {
+        this.errnum = 10;
+        super("Out of range");
+    }
+}
 class OutOfDATA : SmileBasicError
 {
     this()