A metal plate comprises two flat wall forming portions 89, 90 connected together by a joint portion 88, a plurality of reinforcing wall forming portions 83, 84 upwardly projecting from each of the wall forming portions 89, 90 integrally therewith, and a side wall forming portion 81, 82 formed at each of opposite side edges of the plate and upwardly projecting therefrom integrally therewith. A projection 85 is formed on the upper end of the reinforcing wall forming portion 83 on the flat wall portion 89, and a recess 86 for the projection 85 to fit in is formed in the upper end of the wall forming portion 84 to be butted against the portion 83 and provided on the other flat wall portion 90. The metal plate satisfies the relationships of: A>a, A/a.ltoreq.1.5, B/b.ltoreq.1.5, C/c.ltoreq.1.5, and D/d/.ltoreq.1.5 wherein A is the cross sectional area of the projection 85, B is the height of the projection 85, C is the maximum width of the projection 85, D is the width of upper end of the projection 85, a is the cross sectional area of the recess 86, b is the depth of the recess 86, c is the maximum width of the recess 86, and d is the width of an opening of the recess 86.