Abstract: A metal material is rolled to form a strip plate material having a middle part and thinner thickness-deviation portions on both sides of the middle part. After that, this strip plate material is bent and deformed in its width direction by roll forming, thus forming a bent and deformed portion in the middle part. The processing method includes a rolling step and a roll forming step, which enables the metal material to be processed economically using relatively simple steps. No annealing step is included and therefore an excellent final product is obtained. Particularly, the rolling step and the roll forming step are carried out successively, so that the roll forming can be carried out with high efficiency using processing heat generated in the rolling step. The bent and deformed portion is thick and the thickness-deviation portions are thinner, which contributes to the reduction in weight of a final product while the same strength as that of a conventional product is maintained.