A gear mounting structure includes a worm shaft, a gear, and an elastic member. The worm shaft includes a shaft body, on the outer surface of which is formed intersecting spiral grooves, and a gear locking portion with a non-circular cross section, on at least a portion of the outer surface of which is formed a curved surface portion and a planar portion. The gear includes a gear body, on the outer surface of which are formed teeth, and to which the handle rotation is transmitted, and a boss portion protruding from the gear body and having an engagement portion engaging the gear locking portion. The elastic member is mounted so as to abut the outer surface of the boss portion of the gear, and at least a portion of the curved surface portion of the gear locking portion.