Abstract: A decelerating-side tooth surface of each tooth of a drive gear contacts a corresponding one of teeth of a driven gear to transmit torque when torque transmitted between the drive gear and the driven gear is maximum, and is formed as follows. The decelerating-side tooth surface protrudes toward the corresponding tooth of the driven gear and has a circular arc curvature to a facewidth direction of the corresponding tooth. The decelerating-side tooth surface is formed so that a center of the circular arc curvature is located apart in a facewidth direction of the tooth of the drive gear from a plane perpendicular to the facewidth direction in a middle in the facewidth direction toward an opposite side from a portion of the decelerating-side tooth surface that contacts the corresponding tooth of the driven gear at the time when the transmitted torque is maximum.