PURPOSE: To prevent the concentrative excess load by unbalanced load and to sufficiently improve the fatigue strength by setting pitches and twist angles in the tooth tip part of a tool tooth form and in the tooth part of the opposite gear impressed with this tool tooth form so as to mutually equalize.

CONSTITUTION: The pitches and the twist angles are set so as to mutually equalize into the base part P of the worked gear W having radius rx1 and the tooth tip part E of the tool tooth form 2 having radius rx2. Therefore, as the contacting point Q between the worked gear W and tool tooth form 2 is faithfully shifted in the arrow mark S2 direction along the center line in the width direction of a tooth groove D2 of the worked gear W in the condition of engaging with the worked gear W to the tool 1, the round chamfering part 5 of the tooth tip part E in the tool tooth form 2 is uniformly contact-pressed with the tooth base parts P, P at both sides of the tooth groove D2 and made to be impressed. Therefore, the impressed load is not concentratively applied to one side of the tooth base part in the uneven condition, and the residual stress is developed by the further large impressed load to improve the fatigue strength of the tooth base part in the gear.