To provide a worm wheel which is compact and low cost and excellent in durability.

A tooth face 23 of the worm wheel 21 includes a main tooth face 26 and an extending tooth face 27, the main tooth face 26 is arranged inside of reference circle C2 having a radius r2 a value constituted by multiplying the module m of the worm wheel 21 by a predetermined coefficient α larger than a radius r1 of a pitch circle C1 of the worm wheel 21. The extending tooth face 27 extends from the main tooth face 26 to the outside of the reference circle C2 and is inclined to the inside of the tooth part 24 relative to the extending face P1 of the main tooth face 26.