A reciprocating mechanism 8 includes a first supporting member 24, a second supporting member 25, and a biasing member 26. The worm shaft 20 includes a shaft portion 20a, a first shaft supporting portion 20b having a spherical portion 20e formed in a convex spherical shape at a distal end, and a second shaft supporting portion 20c. The first supporting member 24 includes a first bottom portion 24a abutting the spherical portion 20e. The second supporting member 25 includes a second bottom portion 25a abutting a distal end of the second shaft supporting portion 20c. The biasing member 26 biases at least one of the first supporting member 24 and the second supporting member 25 in an axial direction.
