diff --git a/ChangeLog b/ChangeLog
index e99ae3b..aba93a5 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,14 @@
+Mon May 18 04:52:25 2009  Minero Aoki  <aamine@loveruby.net>
+
+	* net/loveruby/cflat/compiler/TypeChecker.java: allow ptr-ptr.
+
+	* net/loveruby/cflat/compiler/IRGenerator.java: implement ptr-ptr.
+
+	* net/loveruby/cflat/type/VoidType.java: VoidType#size is 1.
+
+	* net/loveruby/cflat/ast/ExprNode.java: new helper method
+	  #baseType.
+
 Mon May 18 03:50:55 2009  Minero Aoki  <aamine@loveruby.net>
 
 	* net/loveruby/cflat/sysdep/x86/CodeGenerator.java (optimize):
diff --git a/net/loveruby/cflat/ast/ExprNode.java b/net/loveruby/cflat/ast/ExprNode.java
index 9d72655..0fd1da4 100644
--- a/net/loveruby/cflat/ast/ExprNode.java
+++ b/net/loveruby/cflat/ast/ExprNode.java
@@ -11,6 +11,7 @@ abstract public class ExprNode extends Node {
     protected Type origType() { return type(); }
 
     public long allocSize() { return type().allocSize(); }
+    public long baseSize() { return type().baseType().size(); }
 
     public boolean isConstant() { return false; }
     public boolean isParameter() { return false; }
diff --git a/net/loveruby/cflat/compiler/IRGenerator.java b/net/loveruby/cflat/compiler/IRGenerator.java
index e1fa6b6..44cea3f 100644
--- a/net/loveruby/cflat/compiler/IRGenerator.java
+++ b/net/loveruby/cflat/compiler/IRGenerator.java
@@ -565,13 +565,21 @@ class IRGenerator implements ASTVisitor<Void, Expr> {
         Expr right = transformExpr(node.right());
         Expr left = transformExpr(node.left());
         if (node.operator().equals("+") || node.operator().equals("-")) {
-            if (node.left().type().isPointer()) {
-                right = new Bin(ptrDiffType(), Op.MUL,
-                        right, ptrDiff(node.left().type().baseType().size()));
+            if (node.left().isPointer() && node.right().isPointer()) {
+                Expr tmp = new Bin(asmType(node.type()),
+                        Op.internBinary(node.operator(),
+                                        node.type().isSigned()),
+                        left, right);
+                return new Bin(ptrDiffType(), Op.S_DIV,
+                        tmp, ptrDiff(node.left().baseSize()));
             }
-            else if (node.right().type().isPointer()) {
+            else if (node.left().isPointer()) {
+                right = new Bin(ptrDiffType(), Op.MUL,
+                        right, ptrDiff(node.left().baseSize()));
+            }
+            else if (node.right().isPointer()) {
                 left = new Bin(ptrDiffType(), Op.MUL,
-                        left, ptrDiff(node.right().type().baseType().size()));
+                        left, ptrDiff(node.right().baseSize()));
             }
         }
         return new Bin(asmType(node.type()),
diff --git a/net/loveruby/cflat/compiler/TypeChecker.java b/net/loveruby/cflat/compiler/TypeChecker.java
index 31e88b8..f2843b6 100644
--- a/net/loveruby/cflat/compiler/TypeChecker.java
+++ b/net/loveruby/cflat/compiler/TypeChecker.java
@@ -288,25 +288,25 @@ class TypeChecker extends Visitor {
      *   * integer + pointer
      *   * integer - integer
      *   * pointer - integer
+     *   * pointer - pointer
      */
-    protected void expectsSameIntegerOrPointerDiff(BinaryOpNode node) {
-        if (node.left().type().isPointer()) {
-            if (node.left().type().baseType().isVoid()) {
-                wrongTypeError(node.left(), node.operator());
+    private void expectsSameIntegerOrPointerDiff(BinaryOpNode node) {
+        if (node.left().isPointer() && node.right().isPointer()) {
+            if (node.operator().equals("+")) {
+                error(node, "invalid operation: pointer + pointer");
                 return;
             }
+            node.setType(typeTable.ptrDiffType());
+        }
+        else if (node.left().isPointer()) {
             mustBeInteger(node.right(), node.operator());
             // promote integer for pointer calculation
             node.setRight(integralPromotedExpr(node.right()));
             node.setType(node.left().type());
         }
-        else if (node.right().type().isPointer()) {
+        else if (node.right().isPointer()) {
             if (node.operator().equals("-")) {
-                error(node, "invalid operation integer-pointer");
-                return;
-            }
-            if (node.right().type().baseType().isVoid()) {
-                wrongTypeError(node.right(), node.operator());
+                error(node, "invalid operation: integer - pointer");
                 return;
             }
             mustBeInteger(node.left(), node.operator());
diff --git a/net/loveruby/cflat/type/VoidType.java b/net/loveruby/cflat/type/VoidType.java
index eb62135..b67f226 100644
--- a/net/loveruby/cflat/type/VoidType.java
+++ b/net/loveruby/cflat/type/VoidType.java
@@ -7,7 +7,7 @@ public class VoidType extends Type {
     public boolean isVoid() { return true; }
 
     public long size() {
-        throw new Error("VoidType#size called");
+        return 1;
     }
 
     public boolean equals(Object other) {
diff --git a/test/ptrdiff.cb b/test/ptrdiff.cb
new file mode 100644
index 0000000..02ab211
--- /dev/null
+++ b/test/ptrdiff.cb
@@ -0,0 +1,16 @@
+import stdio;
+
+int
+main(int argc, char** argv)
+{
+    int x, y;
+    void *p = &x;
+    void *q = &y;
+
+    printf("%d", q - p);
+    printf(";%d", q - (p + 1));
+    printf(";%d", q - (1 + p));
+    printf(";%d", q - (p - 1));
+    puts("");
+    return 0;
+}
diff --git a/test/test_cbc.sh b/test/test_cbc.sh
index e8efaa7..3c6ba89 100644
--- a/test/test_cbc.sh
+++ b/test/test_cbc.sh
@@ -209,6 +209,7 @@ test_22_pointer() {
     assert_out "1;2;3;4;5;6;77;78" ./ptrmemb
     assert_out "7" ./ptrmemb2
     assert_out "OK;OK;OK;OK" ./addressof
+    assert_out "-4;-5;-5;-3" ./ptrdiff
     assert_compile_error deref-semcheck1.cb
     assert_compile_error deref-semcheck2.cb
     assert_compile_error deref-semcheck3.cb