PROBLEM TO BE SOLVED: To prevent a constriction or a wrinkle from occurring in a corner part when a front plate is manufactured by bending and processing a front blank, and to maintain airtightness between the corner part and a top plate in a good state when the top plate is mounted at the upper end part of the front plate.SOLUTION: In a cold rolling plate sectioned with a step, a bent corner part orthogonal to the ridge line of the step has a notch at an upper end, and two or more recessed embossment beads parallel with the ridge line of the step on a side surface.
