To provide a system for monitoring the inside of a device at all the time in spite of a transmission line state by respectively switching a transmission line clock and a transmission frame pulse to be inputted to a signal processing part to an intra-device reference clock and an intra-device reference frame pulse when either input clock interruption or out-of-frame synchrobnism is detected through a clock interruption detecting circuit or frame synchronizing circuit.

When the input clock interruption is detected by a clock interruption detecting circuit 5, a clock interruption detecting alarm 14 is inputted through an OR circuit 23 to selector parts 21 and 22 and when the out-of-frame synchronism is detected by a frame synchronizing circuit 9, an out-of-frame synchronism alarm 15 is inputted through the OR circuit 23 to the selector parts 21 and 22. When the clock interruption detecting alarm 14 or out-of-frame synchronism alarm 15 is received, the selector parts 21 and 22 respectively switch the clock and pulse to be inputted to a signal processing part 10 to the intra-device reference clock and intra-device reference frame pulse.