PURPOSE: To form a stepped box-shaped component parts hard to generate breakage at its stress concentrating part by bending a flange part into a prescribed angle after casting the flange part of a component part by providing an inclination angle to the flange part, in casting the stepped box-shaped part.

CONSTITUTION: In casting a stepped box-shaped component part 1, the parts 1 is cast by providing a ceratain angle θ inclining by angle θ with respect to an angle of a flange part 101 of the parts 1 after finishing a work to the part 101, and the cast parts 1 is bent in a hot state preferably at a temperature of about 400W1,300°C by a method of using a flat punch 11, etc. so that the angle of the part 101 is bent to an angle at the time after finishing the forming. Thus the structure at a stress concentrating part such as a corner part is strengthened to prevent breakage. A level of 10W45° said inclination angle is practically preferable, and it is required that an extent of spring back is added to the extent of work when the bending is performed in a cold state.