A detection target voltage is divided by a voltage divider circuit wherein a large number of resistor circuits, each of which is formed by a plurality of resistors being connected in parallel, are connected in series, and the voltage is detected. The output of a multiple voltage detector circuit formed of an operational amplifier, and the like, connected to three arbitrary places in the voltage divider circuit is input into a CPU twice, when the switch is in an on-state and when it is in an off-state. The CPU, based on the two measured voltages, can determine that trouble has occurred when there is, for example, trouble such as a short circuit or disconnection of a resistor of the resistor circuits related to the places connected to the three places in the voltage divider circuit.