A preformed product of a crankshaft is formed in a shape smaller than a shape of a cavity of a separatable die set 1103 composed of an upper die and a lower die and is disposed in the cavity of the die set 1103 as a material M. The cavity of the die set 1103 matches a required shape of the crankshaft. When the material M is disposed in the cavity of the die set 1103, a clearance is formed between the material M and the cavity, and when a side-forming punch 1000P is inserted into the material M, a hollow hole is formed in the material M and the cavity is filled with the material M. As a result, not only can weight saving be achieved by forming the hole in the crankpin of the crankshaft, but also degradation of dimensional accuracy by forming the hole can be avoided.