A plastic helical gear has a three-dimensional tooth surface modification section on a tooth surface of each of involute-shaped teeth. The three-dimensional tooth surface modification section is a combined surface of a tooth top modification surface and an arc crowning surface. The tooth top modification surface is reduced in tooth thickness from a position between a tooth top and a tooth root toward the tooth top. The arc crowning surface is reduced in tooth thickness from a position between one end in the tooth width direction and the other end in the tooth width direction toward both ends in the tooth width direction. A line extending from an intersection point P0 between the starting position of the tooth top modification surface and the apex position of the arc crowning surface to the tooth root is aligned with a line on the tooth surface of the involute-shaped tooth.