PURPOSE: To improve the productivity of an object to be worked by working the object by using a working roller on which plural groove shaping parts provided with a projection for forming a groove, and a flat part having no projection are provided laternately in the axial direction, and cutting the flat part in a longitudinal direction.

CONSTITUTION: A working roller uses that which is used for one face working, and consists of a work roller 2 for forming a groove, a receiving roller 3 and a backup roller 4. The work roller 2 consists of a groove forming part 21, a flat part 22, a flange part 23 and a rotation axis part 20. Also, on the groove forming part 21, many projections 211 are provided, and by this work roller, vertical stripes 111 are formed in the longitudinal direction of a steel plate 10. Subsequently, by executing the working by the second work roller for forming a groove in the horizontal direction, the steel 10 having the vertical stripes 111 and horizontal grooves 112 is obtained. Since the backup roller is provided, a thin steel plate is brought to groove working with high accuracy. Next, a flat unworked part 12 is cut in the longitudinal direction by a slitter. In such a way, the productivity is improved and working can be executed uniformly with a high yield.