Abstract: A power transmission device having: a case formed by assembling a first case member and a second case member; and a shaft member supported by the case, the power transmission device comprising: a protrusion provided to the first case member and protruding in the axial direction along to the direction of assembling the case; and a recess provided to the second case member and recessed in the axial direction. The shaft member is supported by the protrusion in the recess.

Abstract: The gear mechanism includes a gear shaped to have a bottom land, a first tooth flank connected to the bottom land, a second tooth flank connected to the bottom land, a first tooth face connected to the first tooth flank, and a second tooth face connected to the second tooth flank. The bottom land is curved, a radius of curvature of the first tooth flank is greater than a radius of curvature of the second tooth flank, and parts of the first tooth flank and the second tooth flank connected to connection points with the bottom land are curved such that a distance of each of the parts from a shaft center of the gear decreases toward the connection points and that a gradient of each of the parts decreases toward the connection points.