To detect both fusing and leakage of a main relay in a simple circuit configuration by using a fusing detection circuit also for leakage detection.

A power supply device includes: a battery 1 comprising a plurality of unit cells 10 connected; positive and negative main relays 2 connecting positive and negative sides of the battery 1 to a load 30; a control circuit 3 for controlling on/off of each main relay 2; a positive fusing detection circuit 4A for detecting a voltage difference between a junction 11 of the unit cells 10 and an output side of the positive main relay 2A to detect fusing of the positive main relay 2A; and a negative fusing detection circuit 4B for detecting a voltage difference between the junction 11 of the unit cells 10 and an output side of the negative main relay 2B to detect fusing of the negative main relay 2A. The positive fusing detection circuit 4A and the negative fusing detection circuit 4B detect output voltages of the main relays 2 while the control circuit 3 exerts control so as to turn off the main relays 2, and detect leakage of the battery 1 from the detected voltages.