In the helical gear, an opening perimeter wall includes a first arc side face located at the leading end side in the direction of rotation, concave towards the leading end side in the direction of rotation, and having a distance from the arc center as a first radius. The first arc side face includes a side face at a side of the rotation center of the hub, located at position with a distance from the arc center differing from the length of the first radius, on a verge of a virtual arcuate line centered about the rotation center, and passing through the arc center.