PURPOSE: To realize a sure resetting action, by securing the AND conditions between the signal which fixes an output port at an H level and a synchronizing signal when the working of a retriggerable monostable multivibrator is stopped and retriggering the multivibrator for the input of a resetting signal to a microcomputer.

CONSTITUTION: An H level set when the working of a retriggerable monostable multivibrator 2 is stopped is supplied to a 1st AND circuit 12 together with an external synchronizing signal. Then the output of the circuit 12 and the output of a microcomputer 1 are supplied to one of both inputs of a 2nd AND circuit 15 via an OR circuit 14. While the output of the circuit 12 is outputted to the other input of the circuit 15 via a delay circuit 13 and a NOT circuit 16. Then the output of the circuit 15 is supplied to the multivibrator 2 and the circuit 15 applies the pulse output of an H level to the multivibrator 2 by the first resetting pulse received from an AND circuit 4 even though the microcomputer 1 is unresettable.