A vehicle power supply device performs highly accurate determination of degradation of the capacitor unit as follows. Internal resistance and capacitance are corrected according to the temperature of capacitor unit during charge or discharge, thereby obtaining a corrected calculated value if capacitance. The corrected calculated value of capacitance is substituted into a degradation determination formula, which is pre-calculated according to the temperature, thereby calculating the standard value corresponding to the temperature detected by temperature sensor. When the standard value is equal to or less than the corrected calculated value of internal resistance, capacitor unit is determined to be degraded.