To provide a hydroform process having high safety which is hardly affected by the variance of a material.

In the hydroform process, the internal volume Vz during the process of a product which can be calculated by formula Vz =((V0+Vi0)×(R-P)/R)-Vip is controlled as the shaft pressing is advanced, where V0 denotes the volume inside a metallic pipe before starting the hydroforming; Vi0 denotes the total value of the volume inside a booster machine and the volume inside a high-pressure pipe before starting the hydroforming; Vip denotes the total value of the volume inside the booster machine and the volume inside the high-pressure pipe during the hydroforming; R denotes the volume elasticity of a hydroforming liquid, and P denotes the internal pressure during the hydroforming. By controlling the internal volume Vz of the product which is the pipe expansion value of the product as the shaft pressing is advanced, the high internal pressure is naturally loaded to a material having high strength and large thickness, and the low internal pressure is loaded to a material having low strength and small thickness, and any defect caused by the variance of the material can be suppressed.

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