PURPOSE: To provide service with high convenience by sending an event received via a channel D from a private branch exchange to a DTE, and sending reception data from the DTE to the private branch exchange via the channel D as the event.

CONSTITUTION: When a digital telephone set 17 receive data from the private branch exchange 27, the data on channel B is multiplexed with the data on channel D, and they are transmitted to a communication interface 20. An event disassembling means 25 receives only the data on channel D from the interface 20, and forms a D-channel frame. Thence, the disassembling means 25 sends out frame data to the digital telephone set 17 and the DTE 28 by controlling a telephone set control means 24 and a DTE interface means 21. When the telephone set 17 transmits the D-channel frame to the exchange 27, an event synthesis means 26 discriminates whether or not a request is a transmission request from the DTE 28 or the telephone set 17, and transmits it to the exchange 27 by controlling the communication interface 20.