A tank mounting structure adopts a new approach against a force of moving downward a high-pressure tank transversely mounted on a vehicle. A tank mounting assembly has a winding strap and a restraining strap located on an upper side and a lower side of a high-pressure tank to transversely fix the high-pressure tank between a frame and a frame. This structure causes the high-pressure tank to be transversely mounted on the vehicle. A winding strap is wound on the high-pressure tank. A restraining strap extended from a fixing bracket of the winding strap is laid on the winding strap to be placed directly on an upper end portion of a circumferential wall of the high-pressure tank and is fastened to the frame.