PURPOSE: To eliminate useless synchronization check at no connection of a digital telephone set and meaningless power supply reset control by providing a current detection section in a feeding loop and applying current check before the synchronization check.

CONSTITUTION: When no current is detected by a current detection section 28, a 1st control section 22 omits the communication control relating to a digital telephone set 31 afterward and implement only the current check. When a current is detected, the synchronization of a 1st data communication section 24 is checked and when out of synchronism takes place, a feeding changeover means 25 is turned off and then turned on to apply power supply reset to the digital telephone set 31 and when synchronization is taken but no communication reply exists, a reset record is set in a communication frame and the result is sent to a 2nd data communication section 34 via the 1st data communication section 24, a reset signal generating section 38 generates a reset signal to reset a 2nd control section 22. Thus, meaningless synchronization check and power supply reset at no connection of the digital telephone set are avoided.