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 that rotatably supports one end of the worm shaft ; an urging member that abuts against an outer peripheral surface of the bearing and urges the worm shaft toward the worm wheel via the bearing ; 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 that rotatably supports one end of the worm shaft ; an urging member that abuts against an outer peripheral surface of the bearing and urges the worm shaft toward the worm wheel via the bearing ; 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 . <EOS>