A spring hook structure includes: a torsion coil spring including a coil portion and a first arm and a second arm extending from each side of the coil portion; and a rotation support pin by which a cap is rotatably supported on an exterior case. The rotation support pin is inserted into the coil portion. The first arm is hooked on a first hook surface of the exterior case, and the second arm is hooked on a second hook surface of the cap. At least one of the exterior case and the cap is provided with a regulation wall at a position not interfering with the arm moving along the hook surface due to flexural deformation of the coil portion but interfering with the arm moving along the hook surface due to flexural deformation of the arm.