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