According to the hydroforming method by the invention, when slots with the aspect ratio of 3 or more are pierced onto the bulge-formed portion made by the hydroforming, even the piercing can be carried out within a series of processing steps in the hydroforming, thereby eliminating the piercing operation by means of the cumbersome machining process such as milling and ensuring an excellent slot configuration. Thus, the hydroformed parts by the invention are best suited for the automobile parts etc. where various piercing operations are required, and the mold used for the hydroforming method by the invention can be widely employed for processing the automobile parts etc., whereby the invention can be applied for processing the parts in automobiles and industrial machineries as well.