A worm reducer comprising: a worm shaft; a worm wheel that meshes with the worm shaft; a housing including: a tubular shaft housing portion that houses the worm shaft; and a wheel housing portion that houses the worm wheel; a bearing (22, 23) that rotatably supports one end of the worm shaft; an urging member that abuts against an outer peripheral surface of the bearing (22, 23) and urges the worm shaft toward the worm wheel via the bearing (22, 23); and a lubricant housed in an internal space of the housing, wherein the worm shaft includes: a tooth flank forming portion where a tooth flank is formed; and a non-tooth flank forming portion formed between the one end and the tooth flank forming portion, and the worm reducer is characterized in that the shaft housing portion includes: a substantially straight portion having a substantially constant bore diameter and extending in an axial direction of the worm shaft; and a reduced diameter portion arranged on the one end side with respect to the substantially straight portion and having a bore diameter reduced from a position that overlaps the tooth flank in the axial direction of the worm shaft to a position that overlaps the non-tooth flank forming portion in the axial direction of the worm shaft. a worm shaft; a worm wheel that meshes with the worm shaft; a housing including: a tubular shaft housing portion that houses the worm shaft; and a wheel housing portion that houses the worm wheel; a tubular shaft housing portion that houses the worm shaft; and a wheel housing portion that houses the worm wheel; a bearing (22, 23) that rotatably supports one end of the worm shaft; an urging member that abuts against an outer peripheral surface of the bearing (22, 23) and urges the worm shaft toward the worm wheel via the bearing (22, 23); and a lubricant housed in an internal space of the housing, wherein the worm shaft includes: a tooth flank forming portion where a tooth flank is formed; and a non-tooth flank forming portion formed between the one end and the tooth flank forming portion, and a tooth flank forming portion where a tooth flank is formed; and a non-tooth flank forming portion formed between the one end and the tooth flank forming portion, and the worm reducer is characterized in that the shaft housing portion includes: a substantially straight portion having a substantially constant bore diameter and extending in an axial direction of the worm shaft; and a reduced diameter portion arranged on the one end side with respect to the substantially straight portion and having a bore diameter reduced from a position that overlaps the tooth flank in the axial direction of the worm shaft to a position that overlaps the non-tooth flank forming portion in the axial direction of the worm shaft. a substantially straight portion having a substantially constant bore diameter and extending in an axial direction of the worm shaft; and a reduced diameter portion arranged on the one end side with respect to the substantially straight portion and having a bore diameter reduced from a position that overlaps the tooth flank in the axial direction of the worm shaft to a position that overlaps the non-tooth flank forming portion in the axial direction of the worm shaft.