PURPOSE: To improve manufacturing efficiency by previously welding plural number of reinforcing plates to a blank material of bottom plate main body and bending the blank material through the reinforcing plates by a roll from its fixed end to the other end in a state of fixing said fixed end to one end of a bending die having an outer surface shape corresponding to a finished shape of the bottom plate main body.

CONSTITUTION: Seven reinforcing materials 2, for instance, are welded to a blank material 1' of bottom plate main body formed into a prescribed flat plate shape by cutting to obtain a blank material A' of bottom plate. One end of the material A' is fixed to one end of a bending die 12 having an outer surface shape corresponding to a finished shape of the bottom plate main body. In this state, a freely rotatable bending roll 8 is moved along the die 12 from the fixed end of the material A' to its other end while applying a given pressure to it through the reinforcing materials 2. Accordingly, the material A' is bent along the die 12 to form a bottom plate.