A power supply device includes: a power supply; a first contactor and a second contactor that are connected between the power supply and a load; a voltage detection circuit that detects a voltage between a load side of the first contactor and a power supply side of the second contactor; and a control unit that sets the contactors to an open/closed state. In a state in which electrical power has not been supplied to the load, a voltage which is greater than zero and less than a voltage at the power supply is applied between a positive electrode and a negative electrode on the load side; and after controlling the contactors to an open state, the control unit makes a decision as to whether or not one of the contactors is welded based upon detection results of the voltage detection circuit.