There is disclosed a private branch exchange in which an SIP terminal is capable of receiving great variety of services provided by the private branch exchange without upgrading version of software of the SIP terminal itself. A standard SIP protocol control module 16, an expanded SIP protocol a control module 17, and an expanded SIP protocol b control module 18 are disposed for SIP system protocol corresponding terminals, convert signals (requests) from the SIP system protocol corresponding terminals into signals of a protocol inside an IP-PBX 1 to transmit the signals to a PBX basic connection process providing module 12, and convert a signal (responses to the request, or a request to the SIP system protocol corresponding terminal) from the PBX basic connection process providing module 12 into a signal of an SIP system protocol to transmit the signal to the SIP system protocol corresponding terminal.