PURPOSE: To improve durability without lowering impact strength of the corner part of a tooth bottom, by a method wherein, by polishing the corner part of the tooth bottom after a gear cutting work, surface coarseness thereof is decreased to a value lower than that of a portion around a pitch circle, and cementation quenching is then effected.

CONSTITUTION: In manufacture of the differential gears of modules 5 and 5 by means of, for example, a material SNCM 420H, at a point of time when a gear cutting work is completed, surface coarseness of a tooth part, i.e., including a part 2 around a pitch circle, is kept approximately at 0.7μm or more, and during a subsequent polishing work, surface coarseness of a corner part 3 of a tooth bottom and a tooth bottom 4 is decreased approximately to 0.2μm or less. Thereafter, cementation quenching and temperating are applied to form a cementation layer 1A. In the cementation layer 1A, an effective depth t1 of the part 2 around the pitch circle is approximate 0.8 mm, and an effective depth t2 of the corner part 3 of the tooth bottom and the tooth bottom 4 is approximate 0.4mm. This constitution improves resistance to wear of the part around the pitch circle of the tooth part without lowering impact strength of the corner part of the tooth bottom, and enables improvement of durability of a gear.