A pressure angle of the worm is set to be greater than a pressure angle of the worm wheel so that a maximum number of meshing teeth becomes smaller than or equal to n (where n is a natural number) with respect to the speed reduction mechanism in which a number of meshing teeth is always n+1 or changed between n+1 and n by rotation under a condition in which the pressure angles of the worm and the worm wheel are the same with respect to each other.