A vehicle power supply device has a capacitor unit including capacitors storing auxiliary power; a temperature sensor; a charge circuit; a capacitor-unit current detector; a capacitor-unit voltage detector; a controller; a storage; and a determination unit. The determination unit determines the degradation level of the capacitor unit based on at least one of an internal resistance standard value and a corrected calculated value of capacitance. This structure determines degradation of the vehicle power supply device more accurately than ever, so that the device can be used until the end of the actual life.