An SIP proxy server 1 includes a forking termination unit 21. The forking termination unit 21 is configured to: in response to receiving an Initial INVITE request message from a originating-side network 6, transmit the Initial INVITE request message to an incoming-side network 7, and in response to receiving an 18X response message at the first time from the incoming-side network, transmit the 18X response message received at the first time to the originating-side network 6. The forking termination unit 21 establishes Early dialog #0 between the SIP proxy server and the originating-side network 6 and establishes Early Dialog #1 that is compatible with an incoming-side terminal 3 that is a transmission source of an 18X response message received at the first time. In response to receiving 18X response message at the second time from the incoming-side network 7, the forking termination unit 21 establishes Early Dialog #2 that is compatible with the incoming-side terminal 3 that is a transmission source of the 18X response message received at the second time.