Hydraulic forming process and hydraulic forming device

Hydraulic fluid (A) is sealed in one side of a workpiece (11) provided between lower die (21) and upper die (22). The movable die is pressed to increase the pressure of the fluid, such that the workpiece is deformed towards other side opposite to the side sealed with fluid. Independent claims are also included for the following: (1) hydraulic forming device; and (2) metal separator for fuel cell.