A power feeding plug locking device that locks a power feeding plug to a power feeding inlet. The power feeding inlet is arranged in a machine including a door that can be locked. The locking device includes a lock mechanism of which operational state is shifted between a lock state and an unlock state. The lock mechanism prohibits removal of the power feeding plug from the inlet in the lock state and allows removal of the power feeding plug from the inlet in the unlock state. An operation unit is operated when switching the operational state of the lock mechanism between the lock state and the unlock state. A door lock state confirmation unit confirms whether the door of the machine is locked. A controller controls the operational state of the lock mechanism based on the operation of operation unit whether the door is locked.