An outer wheel element (16a) formed in an annular shape with a synthetic resin and having a worm wheel tooth part (19a) on an outer circumferential surface thereof embeds a radially outer end part of an inner wheel element (15a) formed in an annular shape with a metal. An outer circumferential surface of the inner wheel element (15a), which is radially superimposed with an engaging part between the worm tooth part (19a) and a the worm wheel tooth part serves as a cylindrical surface part (24). This realizes a structure capable of suppressing manufacturing error of the worm wheel tooth part engaging with the worm tooth part.