Power supply device and method for making decision as to contactor weld of power supply device

A power supply device includes: a power supply (1); a first contactor (2) and a second contactor (3) that are connected between the power supply (1) and a load (9); a voltage detection circuit (5A) that detects a voltage between a load side of the first contactor (2) and a power supply side of the second contactor (3); and a control unit (6) that sets the contactors to an open / closed state. In a state in which electrical power has not been supplied to the load (9), a voltage which is greater than zero and less than a voltage at the power supply (1) 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 (6) makes a decision as to whether or not one of the contactors is welded based upon detection results of the voltage detection circuit (5A).