PURPOSE: To output a failure detecting signal only when a true asynchronous failure is generated by the constitution that the asynchronous failure is detected when an output signal of a monostable multivibrator triggered by an output of a phase proximity detecting circuit goes to high level for a prescribed period or over.

CONSTITUTION: The monostable multivibrator MM3 is triggered by an output signal 24 of an FF2 and its output signal 25 is shown in Fig. (b). That is, the level goes to L after a period T1 at the initial phase locking and goes continuously to H at the asynchronism failure. The MM33 is triggered by the leading edge of the output 25, and its output 26 goes to H for a period T2. Since the relation of T2>T1 exists, the trailing point of time of the signal 26 at the initial operation comes after the signal 25 goes already to L. Thus, a D-FF34 outputs L as shown in the figure (d). Since the signal 25 is at H continuously at the asynchronism state, an FF34 reads the signal 25 at the time of trailing edge of the output 26, and outputs the signal 25 as a failure detecting signal 27.