The present invention provides a locking device for a waste container that keeps the lid of the container closed when the container is knocked over on its side, as may arise from a sudden jerking or jarring motion, such as by impact with the ground, to prevent spillage of its contents. The locking device allows the lid to open automatically when the waste container is tipped over by a dumping operation. The locking device includes a sliding block slidable between a locked position and an open position, and a pendulum weight detachably attached to a magnet, when detached from the magnet, the pendulum weight blocks the sliding block from sliding to the open position. When attached to the magnet, the pendulum weight allows the sliding block to slide to the open position.