Provided is an unpowered and automatically flushing toilet seat, for a water tank, having detachable and waterproof functions. A loading lever rotates, for the descending movement of a toilet seat, and presses a loading member, thereby enabling enhancement of the transmission efficiency of the force and smooth operation. Since the loading member maintains the loaded state until a toilet paper time and flush control unit presses and releases the loading member, which has been moved and loaded by the pressing of the loading lever, a user can easily acquire time for discarding toilet paper.