At the time for forming an internal gear from a raw material 2 of an internal gear, first, the raw material 2 is externally inserted onto a forming die 3 and fixed to the forming die 3 through a pressing die 4. Next, a forming roll 5 is moved in a direction as indicated by an arrow B of FIG. 6 while rotating the forming die 3. By this, the inner peripheral surface of the raw material 3 is pressed against an outer gear part 3b formed on the outer peripheral surface of the forming die 3 so that an internal gear part 1c is formed on the inner peripheral surface of the raw material 2. The rotating direction of the forming die 3 is selected such that the first end part of the outer gear part 3b located on the front side in the feeding direction of the forming roll 5 is moved ahead in the rotating direction (the direction as indicated by an arrow A of FIG. 6) of the second end part located on the rear side in the feeding direction of the forming roll 5.