A gear mounting structure (20) includes a worm shaft (21), a gear (23), and an elastic member (24). The worm shaft (21) includes a shaft body (25), on the outer surface of which is formed intersecting spiral grooves (25a), and a gear locking portion (26) with a non-circular cross section, on at least a portion of the outer surface of which is formed a curved surface portion (26a) and a planar portion (26b). The gear (23) includes a gear body (28), on the outer surface of which are formed teeth, and to which the handle rotation is transmitted, and a boss portion (30) protruding from the gear body (28) and having an engagement portion (30a) engaging the gear locking portion (26). The elastic member (24) is mounted so as to abut the outer surface of the boss portion (30) of the gear (23), and at least a portion of the curved surface portion (26a) of the gear locking portion (26).本發明之課題在於藉由抑制齒輪與蝸桿軸之間之晃動而提高旋轉感，進而防止於齒輪與蝸桿軸之間產生之聲響。 齒輪組配構造20具備蝸桿軸21、齒輪23、及彈性構件24。蝸桿軸21具有：軸本體25，其於外周面形成有交叉之螺旋狀槽25a；及非圓形剖面之齒輪卡止部26，其於外周面之至少一部分形成有曲面部26a與平面部26b(被卡合部之一例)。齒輪23具有：本體部28，其於外周面形成有齒且傳遞手柄之旋轉；及凸座部30，其具有卡合於齒輪卡止部26之卡合部30a且自本體部28突出形成。彈性構件24可抵接地組配於齒輪23之凸座部30之外周及齒輪卡止部26之曲面部26a之至少一部分。
