A control section (20) of a power supply device calculates a battery voltage on the basis of a capacitor voltage detected by a voltage sensor (17) during precharging that is performed with a main relay section (12) turned off and a precharge relay section (15) turn on and the value of resistance of a resistor (14) of a precharge circuit (13). Then, the control section (20) makes a precharge completion determination on the basis of a changing state quantity of the battery voltage. The control section (20) turns off the precharge relay section (15) in accordance with the precharge completion determination.
