A power plug locking device that prevents unauthorized removal of a power plug from an inlet includes a lock member pivoted about its axis. The lock member is pivoted between a lock position, at which the lock member locks the power plug to the inlet, and an unlock position, at which the lock member releases the power plug from a state locked to the inlet. A manual operation unit is manually operated to pivot the lock member from the unlock position to the lock position. A position holding unit is formed to hold the lock member at the lock position and maintain the power plug in a locked state with the lock member.