Abstract: 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.

Abstract: As to a distance in a radial direction of a rotation shaft from a rim at the hub side to an outer peripheral side face, the outer peripheral side face includes a first side region having a distance located at the most leading end side spot in the direction of rotation of a helical gear longer than a distance at a tail end side spot.

Abstract: 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.

Abstract: As to a distance in a radial direction of a rotation shaft from a rim at the hub side to an outer peripheral side face, the outer peripheral side face includes a first side region having a distance located at the most leading end side spot in the direction of rotation of a helical gear longer than a distance at a tail end side spot.

Abstract: The invention provides a rotatable polygon mixture having a body shaped in a form of polygon. At the axis of rotation of the body is provided a shaft hole, and to each side of said body is provided a mirror surface. The thickness of each center portion of the body corresponding to a portion between the center part of each side and the axis of rotation is made thinner than that of each corner portion corresponding to a portion between the corner part of each side and the axis of rotation to control the flow speed of molding resin.

Abstract: The invention provides a rotatable polygon mirror consisting of (a) cylindrical boss being disposed at the center part of polygon and having a hole therein, (b) side plate being disposed at each side of polygon, the outer surface of the side plate forming a mirror surface, (c) connecting-plate for connecting between the cylindrical boss and the side plate, and rib being radially disposed between the cylindrical boss and each corner of polygon.