diff --git a/include/mcl/window_method.hpp b/include/mcl/window_method.hpp
index cb4fad3..64ff869 100644
--- a/include/mcl/window_method.hpp
+++ b/include/mcl/window_method.hpp
@@ -23,35 +23,33 @@ struct ArrayIterator {
 		, bitSize(bitSize)
 		, w(w)
 		, pos(0)
-		, mask((w == TbitSize ? 0 : (T(1) << w)) - 1)
+		, mask(makeMask(w))
 	{
 		assert(w <= TbitSize);
 	}
+	T makeMask(size_t w) const
+	{
+		return (w == TbitSize) ? ~T(0) : (T(1) << w) - 1;
+	}
 	bool hasNext() const { return bitSize > 0; }
 	T getNext()
 	{
-		if (w == TbitSize) {
-			bitSize -= w;
-			return *x++;
+		if (bitSize < w) {
+			w = bitSize;
+			mask = makeMask(w);
 		}
-		if (pos + w < TbitSize) {
-			T v = (*x >> pos) & mask;
-			pos += w;
-			if (bitSize < w) {
-				bitSize = 0;
+		if (pos + w <= TbitSize) {
+			T v = x[0] >> pos;
+			if (pos + w < TbitSize) {
+				pos += w;
+				v &= mask;
 			} else {
-				bitSize -= w;
+				pos = 0;
+				x++;
 			}
+			bitSize -= w;
 			return v;
 		}
-		if (pos + bitSize <= TbitSize) {
-			assert(bitSize <= w);
-			T v = *x >> pos;
-			assert((v >> bitSize) == 0);
-			bitSize = 0;
-			return v & mask;
-		}
-		assert(pos > 0);
 		T v = (x[0] >> pos) | (x[1] << (TbitSize - pos));
 		v &= mask;
 		pos = (pos + w) - TbitSize;
diff --git a/test/window_method_test.cpp b/test/window_method_test.cpp
index 1b0f702..437ad55 100644
--- a/test/window_method_test.cpp
+++ b/test/window_method_test.cpp
@@ -6,21 +6,24 @@
 
 CYBOZU_TEST_AUTO(ArrayIterator)
 {
-	const uint32_t in[2] = { 0x12345678, 0xabcdef89 };
-	const size_t bitSize = 64;
-	for (size_t w = 1; w <= 32; w++) {
-		const uint32_t mask = uint32_t((uint64_t(1) << w) - 1);
-		mpz_class x;
-		mcl::gmp::setArray(x, in, 2);
-		mcl::fp::ArrayIterator<uint32_t> ai(in, bitSize, w);
-		size_t n = (bitSize + w - 1) / w;
-		for (size_t j = 0; j < n; j++) {
-			CYBOZU_TEST_ASSERT(ai.hasNext());
-			uint32_t v = ai.getNext();
-			CYBOZU_TEST_EQUAL(x & mask, v);
-			x >>= w;
+	const uint32_t in[] = { 0x12345678, 0xabcdef89, 0xaabbccdd };
+	for (size_t bitSize = 1; bitSize <= 64; bitSize++) {
+		for (size_t w = 1; w <= 32; w++) {
+
+			const uint32_t mask = uint32_t((uint64_t(1) << w) - 1);
+			mpz_class x;
+			mcl::gmp::setArray(x, in, CYBOZU_NUM_OF_ARRAY(in));
+			x &= (mpz_class(1) << bitSize) - 1;
+			mcl::fp::ArrayIterator<uint32_t> ai(in, bitSize, w);
+			size_t n = (bitSize + w - 1) / w;
+			for (size_t j = 0; j < n; j++) {
+				CYBOZU_TEST_ASSERT(ai.hasNext());
+				uint32_t v = ai.getNext();
+				CYBOZU_TEST_EQUAL(x & mask, v);
+				x >>= w;
+			}
+			CYBOZU_TEST_ASSERT(!ai.hasNext());
 		}
-		CYBOZU_TEST_ASSERT(!ai.hasNext());
 	}
 }