PURPOSE: To surely detect the running-away state of a CPU by making the clear conditions of a watchdog timer(WDT) severe.

CONSTITUTION: A CPU 6 alternately writes previously decided data into a data discriminating circuit 14 to which a data bus is connected. The output terminal of the data discriminating circuit 14 is connected to input terminals J and K to a JK flip-flop 9 consisting of a pulse generating circuit 15, and the output state of that data discriminating circuit 14 is changed corresponding to the write of the CPU 6. The pulse generating circuit 15 reads the states of those input terminals J and K corresponding to a trigger loaded by the falling edge of a write signal from the CPU 6 to the data discriminating circuit 14 and prepares a clear pulse with respect to a WDT 19.