When streaming transfer of voice or image is performed using a real time streaming protocol (RTSP), streaming transfer of a high bit rate is enabled even in an environment of using network address port translation (NAPT) and network address translation-protocol translation (NAT-PT). When a SETUP message of RTSP is received, a port number having the same number of digits as a client port number notified by a client is assigned from a port number pool, and is registered in an NAPT table. The SETUP message is rewritten with the port number.