A gear box includes a gear box body of a reducer to which a motor is mounted. The gear box body includes a worm, wheel holding section where a worm wheel of the reducer is held, and a motor holding section, where a part of the motor is contained, the motor holding section having a motor mounting opening. The motor holding section is configured such that a bottom of the motor holding section is provided in a region between a line L1 and a line L2. Here, the line L1 passes through the center C of the worm wheel holding section and is perpendicular to the axis line X of a motor shaft. The line L2 is a tangent line passing through an outer edge portion located near a circular outer peripheral wall of the worm wheel holding section on a motor side and is perpendicular to the axis line X of the motor shaft.