Worm wheel, electric power steering system, and manufacturing method for worm wheel

A worm wheel comprising:

• a sleeve having an annular shape and:
• an outer peripheral surface, which is a surface of the sleeve in a radial direction, the outer peripheral surface having a central part, in an axial direction, that projects in the radial direction relative to both ends in the axial direction;
• a first side face which is a surface of the sleeve on a side closer to an injection-mold gate in the axial direction;
• and
• a second side face which is a surface of the sleeve on an opposite side to the first side face in the axial direction;
• and
• a toothed portion provided by injection molding so as to cover a part of the sleeve, the toothed portion having:
• a tooth forming portion provided on the outer peripheral surface;
• a first thick portion provided on the first side face;
• and
• a second thick portion provided on the second side face, the first thick portion having a smaller thickness in the axial direction than the second thick portion.