PURPOSE: To reduce the number of times of the periphery polishing work at the time of manufacturing a side rail by restricting the fluctuation of the width dimension within the length corresponding to one ring of an oil ring compact within the specified value.

CONSTITUTION: Fluctuation of width dimension of a flat steel wire material for side rail of oil ring within the length corresponding to one ring of the oil ring compact is restricted within 8μm. A material 7 to be formed, which is formed by Turk's head rolls as the final stage working through the pressing and a preparatory forming stage for pressing down the roll from the surface and the back surface thereof, is constrained by an upper and a lower bearing rolls 8 from the surface and the back surface thereof so as to prevent the generation of the bend and center thickness, and even in the case where the fluctuation of the cross sectional area is generated, since the surface and the back surface of the material 7 to be formed are constrained by the upper and the lower bearing rolls 8 and the edge thereof is constrained by an edge bearing roller 9, all the fluctuation generated in the working at the final stage is converted to the extension in the longitudinal direction, and the width dimensional fluctuation is restricted.