To confirm a protocol version to perform communication, when a button telephone main device mixedly houses a new communication protocol extension terminal and an old communication protocol extension terminal, to perform mutual communication.

When it is detected that an extension telephone is connected, a first response request command for requesting predetermined response is transmitted to a corresponding extension by using an extension communication protocol having the oldest release date; when the predetermined response corresponding to the first response request command is received, a second response request command for requesting a predetermined response to the extension by using an extension communication protocol having a release date newer than that of the version; and when a predetermined response corresponding to the second response request command is not received, the extension communication protocol of the extension is determined to be a first version.