A gateway apparatus arranged between the mobile circuit switched network and the mobile IMS network, includes a call control unit which converts a control signal output from the mobile circuit switched network into at least one of SIP and SDP to output a signal converted to the mobile IMS network and a conversion unit which receives a line switching protocol for an audio signal output from the mobile circuit switched network. The conversion unit decides, using at least one of SIP and SDP, output from the mobile IMS network and received by the call control unit, on whether the partner terminal is to be connected via the mobile IMS network to the mobile circuit switched network, via the mobile IMS network to a fixed network or via the mobile IMS network to a mobile broadband network, then converts the line switching protocol to one of different protocol, depending on a result of the decision, and outputs the protocol converted signal to the mobile IMS network (FIG. 2).