Forging method and forging die

Forging method of the present invention has a rectangular opening is formed by the wall portion is a rectangular plane, to the work space 45 of the forging mold 20 and a work space 45 to put the workpiece has been formed, rectangular processing adding a placing step of placing the first shape of the workpiece W is a hexahedral shape, the plastic strain to the work by deforming the placed workpiece W to the second shape is a rectangular hexahedral includes a step, characterized by performing the placing step, the treatment process more than once. For example, the work in forging die of the work space in order to pressure variations, it is possible to further ensure the shape stability. Moreover, forging die 20, since it has a plurality of mold parts is fitted to the inner periphery of the outer structure, for example, the outer peripheral side the stress applied to the inner mold 50 upon pressurization of the workpiece W by a plurality of mold parts can be evenly distributed, it is possible to further suppress the destruction of the mold.