PURPOSE: To obtain a rectangular parallelepiped-shaped housing which is tall and has no strain by forming a male die to a prescribed shape by recessing a hollow spherical groove to the top end face of said die thereby balancing and expanding the plastic fluid of a blank material in the long axis and short axis directions.

CONSTITUTION: A blank material 3 is loaded to the inside bottom of a female die 2 for forging having the same plane as the plane of a rectangular parallelepiped-shaped housing and a male die 1 narrower by as much as the thickness of the side wall of said housing is press-fitted into the die hole 2a to produce the housing. The forming land 1e of said die 1 is notched and formed to the tapered surface of which the outside circumferential end edge is directed to the top end face. A hollow spherical groove 1h of the arc surface directed toward the center is recessed to said top end face. The fluid of the material 3 is forced to flow earlier in the longitudinal direction of the top end face than in the transverse direction along the surface of said groove 1h in the stage of pressing the material 3 with the die 1.