A power supply device includes negative-side and positive-side battery blocks serially connected to each other through a resistor element 19. The ends of the element 19 correspond to reference and auxiliary ground points 8A and 8B. A voltage detector 3 detects the voltages of battery modules 2 of a load-driving battery 1. The detector 3 includes an asymmetric resistor circuit that is asymmetrically constructed with respect to the reference point 8A. The detector 3 detects the voltage of the auxiliary point 8B with respect to the reference point 8A in a load driven state. A disconnection detecting circuit 20 compares the detected voltage with a reference voltage of the auxiliary point 8B with respect to the reference point 8A not in the load driven state, and determines that the line 9 is disconnected in the load driven state if the deviation of the detected voltage from the reference voltage falls out of a predetermined range.