The present invention provides a vehicle power supply device that is capable of suppressing deterioration of a capacitor, shortening a precharging time, and suppressing power loss. An ECU estimates a polarization condition of a battery after a key has been switched OFF (IG SW_OFF), and opens a main relay after determining that the polarization condition of the battery has been eliminated. Thus, a voltage difference between a battery voltage and a capacitor voltage when the main relay is subsequently reconnected can be reduced. Hence, even when the capacity of the capacitor is increased, the time required for precharging can be shortened, and energy regeneration and so on can be realized efficiently within a short time period after switching the key ON. In this case, a situation in which the post-release capacitor voltage is maintained at an unnecessarily high level can be prevented even when charging polarization occurs in the battery, and as a result, deterioration of the capacitor can be suppressed.