A gateway unit which relays communication between a call control network and a terminal having no call control function. The gateway unit includes: a session control section which substitutively performs call control processing in call control communication performed by the terminal by way of the call control network and reports a band required for the call control communication to the call control network; and a band adjustment section which determines an updated value of a band of the call control communication based on a communication packet in the call control communication, received from the terminal. The session control section reports the updated value to the call control network as the band required by the call control communication.