A lid locking device of the present invention is provided with a power transmission mechanism which includes a worm gear, a worm wheel, a pressurizing protrusion portion, a pressurized face and an interference avoiding space between a motor and a locking member. The power transmitting mechanism transmits backward-driving power which allows the locking member to move backward to an unlocked position from the motor to the locking member but does not transmit the power from the locking member to the motor and cuts off the power. That is, the backward driving power received by the locking member from an engagement protrusion piece of a lid on closing the lid is cut off by the power transmitting mechanism and not transmitted to the motor.