To eliminate dimensional change caused by water absorption by positively preventing the water absorption in the tooth part of a synthetic resin gear and to effectively prevent the resulting operation failures.

A worm gear reduction mechanism has housings 21, 22 made by a casting method such as aluminum die cast or the like. A worm wheel 12 has a tooth part 12a formed of a synthetic resin. The housings 21, 22 are impregnated with resin. This fills with the impregnation fine pin holes and blow holes formed by the casting method of the housings 21, 22, thereby enhancing the sealing properties of the worm gear reduction mechanism.