PURPOSE: To obtain a high precision forged product with groove by giving inclinations gradually protruding on the inner surfaces parallel to ribs for forming the grooves arranged in a cavity and forming with dies having split type structure divided with the parallel surface to the ribs.

CONSTITUTION: At the time of pushing in a blank 9 with a punch 8, both ends of the blank 9 are abutted on the inclining surfaces of a cavity and plastically deformed. On the way of this deformation, the flow of the blank 9 widened over the bottom surface, is blocked by the penetration of the ribs, and stress P1 is concentrically acted in the inner surfaces of the ribs. However, as the die 1 is divided by the parallel surface to the ribs, the closeness of the divided surface 4 is weakened to absorb the stress P1. On the other hand, both side surfaces of the blank 9 are gathered to the inside along the side surfaces, and a stress P2 is allowed to act on the ribs to keep the balance with the stress P1. These stresses P1, P2 can be adjusted by the inclining angle of both side surfaces and the length of the blank 9, etc.