PURPOSE: To prevent retardation of abnormality detection in a microcomputer, by checking whether an abnormality detection signal is outputted or not when an abnormal signal is inputted to a watchdog part.

CONSTITUTION: An operation button 7 on a console is operated so set a maintenance mode. By this command signal, a microcomputer 3 inhibits the output of a periodic signal S2 in accordance with a program stored in an ROM. Therefore, a watchdog part 4 continues discharging, and an abnormality detection signal S3 becomes 0 after the elapse of a prescribed time. Then, a display part 5 is turned on to display the abnormality, and the microcomputer 3 is reset to stop the operation. Consequently, it is confirmed that the watchdog part 4 outputs the abnormality detection signal in accordance with the abnormal signal of the microcomputer 3. When the operation button 7 is operated again after checking the operation, the microcomputer 3 is returned to the normal operation.