To prolong the product life of a gear forging die for forming a gear, whose tooth traces are inclined to the rotational axis thereof, by preventing cracking of ridges for forming the tooth traces, which are formed on the inner peripheral surface of the die, into helical teeth.

A gear forging die 1 forms a gear 60 having a rotational axis A6 coaxial with an axis A1 of a cavity 20 and having tooth traces inclined to the rotational axis A6, by moving a punch 6 in the direction approaching to a die 2 to pressurizingly deform a billet 50 placed in the cavity 20. In an inner peripheral surface 22 of the die 2, a ridge part 30 is formed, which is projected toward the radial inside of the inner peripheral surface 22 and forges the tooth traces of the gear 60. The longitudinal direction of the ridge part 30 is inclined to the axis A1 of the cavity 20. An opened part side edge surface 31, which is an edge surface in the side of an opened part 21 of the ridge part 30, is composed of a surface vertical to a direction, along which the ridge part 30 is inclined, as viewed from the side of the axis A1 of the cavity 20.