A center axis of a worm shaft is offset in an offset direction with respect to a plane that is orthogonal to a center axis of a worm wheel and that passes through a central position of the worm wheel in a tooth width direction. A bias member biases a second end of the worm shaft in a preloading direction in which a center distance between the worm shaft and the worm wheel decreases. When the worm shaft is configured like a right-hand thread, in a left-handed coordinate system, a direction toward a first end of the worm shaft in an axial direction (a direction toward an electric motor) corresponds to a thumb direction. The preloading direction corresponds to an index finger direction. The offset direction corresponds to a middle finger direction.