To execute radius bending to a construction structural member of U shaped grooved member in combination of a rugged roll forming machine and flat roll forming machine.

The grooved member is subjected to plastic working in wavy shape by a pair of rugged rolls 4 while holding the upper part of left/right side plates 2 of the grooved member formed to a U shape in cross section in the longitudinal direction beforehand, successively, a wave formed part 10 is rolled by a flat roll 11 to generate elongation and to be formed to downward warp, or by reversing this sequence, the grooved member is flat formed by the flat roll 11 while holding the upper part of side plates 2 of the grooved shape member, successively, the grooved member 1 is subjected to plastic working to wavy form by the rugged roll 4 to be formed to upperward warp.