PURPOSE: To attain synchronized transmission between a master station and a slave station by intermitting reception when a fault is detected in a frame signal from the master station and restarting the establishment of synchronism after a silence signal period for a prescribed time is detected.

CONSTITUTION: When one bit of a data bit of the 2nd frame causes bit inversion, a frame synchronism error signal ERR is outputted to a no signal detection section 17 and a window control section 14 in a frame error detection section 16. The window controller 14 resets immediately a reception window RXW. On the other hand, the no signal detection section 17 detects the silence signal period. After the end of the 2nd frame, a no signal period detecting signal SG is fed to the window control section 14 and the reception window RXW is set again.