PURPOSE: To prevent effect due to noise generated at intermission of frame signals by setting a transmission delay time when a frame signal from a slave station is received normally as a reception guard time in receiving the next frame signal.

CONSTITUTION: If any transmission error is detected in the received frame signal, a synchronism error detection signal SERR is outputted to a delay measuring section 5 from a synchronism error detection section 9. A delay measuring section 5 clears the reception guard time used for the reception of the present frame by using the synchronism error detection signal SERR and uses the signal as the initial value, the signal is outputted when the transmission window is reset so as to set the reception window and the transmission delay is measured again.