To improve the dimensional accuracy of a cross section by straightening the cross section of a bar having a special shaped cross section which is slit into a prescribed width and has parts having different thicknesses at least in one or more places in the width direction.

After slitting the bar having the special shaped cross section which is provided with a cross-sectional shape on which thick parts and thin parts are formed in one or more places along the width direction and the cross-sectional shape of which is roll-formed uniformly and continuously in the longitudinal direction of the bar, the straightening of the cross section is performed by passing through at least one cross section straightening stand S which is constituted of upper and lower rolls for finishing the thickness direction of the bar having the special shaped cross section and a pair of side rolls for finishing the width direction.

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