To prevent a gear shaft from falling.

A support member 44 is provided with a temporary stop opening 44a and a slit 44b. The gear shaft 30 is provided with a temporary stop part 42. The temporary stop part 42 is composed of a small-diameter part and a large-diameter part. The small-diameter part 44b is so formed that it can pass through the slit 44b. The large-diameter part, on the other hand, is formed larger than the width of the slit 44b, thinner than the internal diameter of the temporary stop opening 44a. The gear shaft 30 is fitted to the support member 44 by putting the small-diameter part opposite the slit 44b first, moving the gear shaft 30 at right angles to the center-axis direction, and inserting the temporary stop part 42 into the temporary stop opening 44a from the slit 44b. Then the gear shaft 30 is rotated by 90° in the temporary stop opening 44a to put the large-diameter part opposite the slit. The temporary stop part 42 becomes unable to pass through the slit 44b, so the gear shaft 30 never falls from the support member 44.