An urging member urges a first end of a worm shaft toward a worm wheel via a first bearing. The worm shaft includes a tooth flank forming portion and a non-tooth flank forming portion formed between the first end and the tooth flank forming portion. In a housing, a shaft housing portion that houses the worm shaft includes a substantially straight portion and a reduced diameter portion. The substantially straight portion has a substantially constant bore diameter, and extends in an axial direction of the worm shaft. The reduced diameter portion is arranged on the first end side with respect to the substantially straight portion, and has a bore diameter reduced from a position that overlaps tooth flanks in the axial direction to a position that overlaps the non-tooth flank forming portion in the axial direction.