A workpiece is formed into a shape having curvatures in the longitudinal and width directions required for a final closed cross-sectional shape, and bend-facilitating lines are provided at positions corresponding to bent lines in the closed cross-sectional shape. The workpiece formed in the first step is bent in a direction that left and right side wall portions approach each other by clamping the bottom portions between a punch and pad in the plate thickness direction and by pressing a punch into a space between dies. A plug having an outer shape the same as the final closed cross-sectional shape is placed on the bottom portion of the workpiece formed in the second step, and the bottom portion and the left and right side wall portions are bent along the bend-facilitating lines by pressing the bottom portion and the left and right side wall portions against a periphery of the plug.

A method is disclosed for forming a curved portion with a small curvature radius in an original tube while suppressing wrinkles and buckling. The method includes placing the original tube between a first die and a second die, and pressing one or more times on the original tube by bringing the first die and the second die closer together and pressing the first die and second die against the original tube, wherein the original tube has a circular cross-sectional shape in at least a portion thereof, the first die has a first curved surface, the second die has a second curved surface, in one of the pressing, cross-sectioning and bending are simultaneously performed on at least the portion of the original tube by pressing the first die and second die against at least the portion of the original tube; in the cross-sectioning, the circular cross-sectional shape of the original tube is reduced in diameter, and a diameter reduction rate in one time of the pressing is more than 0% and less than 10%.

A method is disclosed for forming a curved portion with a small curvature radius in an original tube while suppressing wrinkles and buckling. The method includes placing the original tube between a first die and a second die, and pressing one or more times on the original tube by bringing the first die and the second die closer together and pressing the first die and second die against the original tube, wherein the original tube has a circular cross-sectional shape in at least a portion thereof, the first die has a first curved surface, the second die has a second curved surface, in one of the pressing, cross-sectioning and bending are simultaneously performed on at least the portion of the original tube by pressing the first die and second die against at least the portion of the original tube; in the cross-sectioning, the circular cross-sectional shape of the original tube is reduced in diameter, and a diameter reduction rate in one time of the pressing is more than 0% and less than 10%.