GEAR

PROBLEM TO BE SOLVED: To increase the rotational torque between a rotary shaft and a wheel gear while sufficiently ensuring the thickness of a boss part without changing the diameter of the boss part of the wheel gear. SOLUTION: The wheel gear 34 is molded on a base end part of the rotary shaft 12 in a gear 10, and the wheel gear 34 is coupled with the base end part of the rotary shaft 12 at a wheel part 36 and the boss part 38. Since the contact surface area of the rotary shaft 12 with the wheel gear 34 is increased with a coupling part with the wheel part 36 of the rotary shaft 12 as a large diameter part 20, the coupling force of the rotary shaft 12 with the wheel gear 34 is increased, and the rotational torque therebetween can be increased. In addition, since a coupling part of the rotary shaft 12 with the boss part 38 is formed to be a small diameter part 26, the thickness (the dimension L) of the boss part 38 can be sufficiently ensured without changing the diameter (the dimension W) of the boss part 38, and cracks of the boss part 38 caused by the rotational torque, the temperature change, etc., applied to the boss part 38 can be prevented thereby.