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 storage stores a correction calculation formula and degradation determination formulas. The correction calculation formula is used to perform the correction calculation of the internal resistance value and the capacitance value in accordance with the temperature in order to obtain a corrected calculated value of internal resistance and a corrected calculated value of capacitance. The degradation determination formulas correspond to the numerical ranges of the corrected calculated value of capacitance and are used to calculate an internal resistance standard value from the corrected calculated value of capacitance. The determination unit determines the degradation level of the capacitor unit based on at least one of the internal resistance standard value and the 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.