PURPOSE: To improve the reliability of a system in terms of coping with the fluctuation of a power source voltage by incorporating a circuit detecting the fluctuation exceeding the rated value of the power source voltage in order to cope with said fluctuation.

CONSTITUTION: If a voltage impressed on a power source terminal 1 drops temporarily to be lower than the regulated value, the detection circuit 5 detects the effect, generates a detection signal and writes it in a storage circuit 6. A CPU outside the titled device reads contents in the circuit 6, or an output terminal 8 outputting said contents to the outside is monitored. Therefore, the occurrence of malfunction is recognized and processing is made. Thanks to such a constitution, malfunction can be previously prevented, or even if malfunction occurs, an operator instantaneously copes with that. Consequently, the reliability of the system can be improved in terms of coping with the fluctuation of the power source voltage.