PURPOSE: To bend workpiece having hollow cross section into various radiuses without filling material into the hollow cross section by filling the hollow cross section of a workpiece with a base mandrel and pushing the mandrel into a bending station under thrust.

CONSTITUTION: A base mandrel 4 which is fixed to a mandrel holding bar 15 is placed within a hollow cross section of a workpiece 11, and the workpiece 11 is pushed into a bending station 5 under longitudinal thrust of a reciprocating pedestal 6 for pushing. Therefore, any material is not necessitated to be poured or filled into the hollow cross section to be bent, and the hollow cross section is bent into various radiuses in a bending zone 14.