A pair of wedge-shaped pieces are provided in an annular space that is defined between an inner circumferential surface of a holding recess portion and an outer circumferential surface of a bearing holder in two position that are symmetrical with each other with respect to a meshing portion between a worm wheel and worm teeth and an imaginary plane α that contains a center axis of a worm in such a way as to fill the annular space. Then, an elastic force is imparted to both the wedge-shaped pieces in a direction in which the pair of wedge-shaped pieces move away from the meshing portion in relation to a circumferential direction.