PURPOSE: To detect the continuously generated omission of reception timing pulses and to enable high-accuracy quality monitor by storing the omission of received pulses as a decrease in the number of received pulses held inside a buffer circuit.

CONSTITUTION: Qn pieces of pulses are set so as to be held at the buffer in a fast-in fast-out IC 1 and with the holding, the count number of a counter 3 is set to the Qn as well. A counter number Qy of a counter 2 is a set value for continuously outputting alarm signals for prescribed time. The transmission line of a synchronous network is provided with the reception timing pulse and a timing pulse in the same cycle as the internal timing of a reception side device and therefore, this device monitors the quality of the transmission line by comparing the internal timing pulse with the received timing pulse. Thus, the intermittent omission of pulse signals which may be missed by the conventional monitoring device and not to generate alarms can be monitored.