PURPOSE: To improve the fatigue strength of the gear subjected to a carburization treatment by meshing a master gear having the strength higher than the above-mentioned gear with the gear and rotating the gears while applying the load of specific times the load under which the part to generate the max. bending stress in the bottom part of the gears begins to yield.

CONSTITUTION: The gear made of steels for machine structural use, etc., is subjected to the carburization treatment and is held for 30 minutes at about 830°C; thereafter, the gear is cooled with oil and is further tempered for about 60 minutes at 180°C. The master gear of the high strength which is likewise subjected to the carburization treatment and is finished with high accuracy by tooth polishing is meshed with the above-mentioned gear and the gears are rotated 1 to 10 turns under the load of 1.15 to 1.35 times the load under which the part to generate the max. bending stress in the bottom part of the gears begins to yield. The excellent fatigue strength which can sufficiently satisfy the required characteristics to the failure of the carburized gear is imparted to the gear.