To provide a hydroforming apparatus capable of accurately positioning metal plates in welding and capable of positioning a common portion both in welding and hydroforming, and a hydroforming method.

A hydroforming apparatus 1 clamps two metal plates 2, 3, superposed on each other, with a pair of molds 11, 12 and injects a medium, which imparts internal pressure, between the metal plates, so that the metal plates are formed into a shape matching the shape of a cavity formed in the molds. Locating holes 4, 5, which penetrate the plates and in which locating pins for welding are inserted and arranged, are formed in the two metal plates. The hydroforming apparatus has sealing mechanisms 30, which are provided on metal plate abutting faces 11A, 12A of the molds and cover the surroundings of the locating holes to seal the medium.