Output voltages of a regulator A, a regulator B and a regulator C are supplied to respective load elements, and are inputted to a representative value deciding logical circuit operating by a private resident power supply. The representative value deciding logical circuit selects the output voltage estimated to be the most abnormal from the output voltages of the regulator A, the regulator B and the regulator C and informs a selector of a selection result. The selector selects only the output voltage from the regulator outputting the output voltage estimated to be the most abnormal, which is selected by the representative value deciding logical circuit, and outputs it to a smoothing circuit. DC current smoothed by the smoothing circuit is quantized by an AD converter and is transferred to a system monitoring processor. Even if a secondary power supply is substituted for the regulator, a circuit which is constituted of the smoothing circuit, the AD converter and the system monitoring processor and which is for monitoring voltage has the same structure as a conventional one.