To provide a rectangular steel product where setting/adjustment and cutting/machining man-hours to a cutter and a finishing machine are reduced and the reduction of an unfavorable influence upon an environment can be achieved at a low cost.

The rectangular steel product is such that out of surfaces which continue in a longitudinal direction, one flat surface is turned black by oxide denaturation and remaining three surfaces are of surfaces to be cut. It is particularly referable that the product is used for a long material, and concretely, it is effective to use the product for a rectangular product with a length of 2 m or longer and a thickness of 20 to 140 mm.