A plane that contains a central axis of a worm shaft (18) and that is orthogonal to a central axis of a worm wheel (19) is arranged at a central position of the worm wheel in a tooth width direction (W) thereof when no load is applied to a worm speed reducer (15). The worm wheel is formed by a cutting operation using a hob with a hob center offset from the central position toward an offset direction corresponding to a first direction along the tooth width direction. A pair of tooth flanks has different pressure angles at the central portion in the tooth width direction.