The large message-memory size has been a major design issue in belief propagation for stereo matching methods. To reduce its size, we proposed an in-place message updating approach which stores new messages into expired ones. Thus, this approach only needs one message memory and a small temporary message memory. Further combining with block based belief propagation, the final architecture only needs 172 KB memory and can reduce internal memory size by 40% when compared with traditional ping-pong buffer approach.