To provide against a short circuit fault of a capacitor to be used for voltage detection and also to make a self diagnosis of the short circuit of the capacitor.

The capacitors C1, C2 are constituted of a series connection of a plurality of capacitors. In a voltage detection mode, a first reference voltage VREF is selected by a reference voltage selection circuit 5, and after setting an electric charge to the first capacitor C1 by turning on switches SW3, SWB4, a cell voltage is detected by turning off the switches SW3, SWB4 and turning on a switch SWB3. In a self diagnosis mode, the first reference voltage VREF is selected by the reference voltage selection circuit 5, and after setting the electric charge to the first capacitor C1 by turning on the switches SW3, SWB4, the switch SW3 is turned off and the fault is diagnosed based on an output voltage of an operational amplifier 4 after selecting a second reference voltage VSLF by the reference voltage selection circuit 5.