A power plug locking device used with a door lock drive source that locks a vehicle door includes a lock mechanism switched between a lock state, in which removal of a power plug from an inlet is prohibited, and an unlock state, in which the removal is permitted. A plug lock drive source performs an unlocking operation that switches the lock mechanism to the unlock state. A plug lock detector, which detects whether the lock mechanism is in the lock state, is connected between the plug lock drive source and an unlock terminal of the door lock drive source. A power supply circuit supplies the plug lock drive source with current and switches the lock mechanism to the unlock state when the door lock drive source is performing an unlocking operation while the plug lock detector is detecting that the lock mechanism is in the lock state.