To improve the dimensional accuracy of the cross section of a strip having a special shaped cross section after heat-treating the strip having the special shaped cross section which is roll-formed by using rolls on which grooves and projected lines having a prescribed shape are formed and parts having different thicknesses in a plurality of places in the width direction.

After annealing the strip having the special shaped cross section on which thick parts and thin parts are formed in one or more places along the width direction, also which has a cross-sectional shape in which trimming grooves are formed on both sides and on which the cross-sectional shape is roll-formed uniformly and continuously in the longitudinal direction of the strip, the strip is passed through between an upper roll which is the same type as an upper roll used when the strip having the special shaped cross section is roll-formed and only the height H1of a projected line part for forming groove parts on both outermost sides is made higher than the height H2of the projected line part for forming the middle thin part and a flat lower roll.

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