A method for forming a closed cross-sectional structure by bending a plate-shaped workpiece at positions of a plurality of bend lines extending in a longitudinal direction, the structure including a bottom portion (2, 3) formed in a central part of the workpiece in a width direction and left (4, 5) and right side wall portions located on both sides of the bottom portion (2, 3) in the width direction, the method comprising: a first step of: press-forming the plate-shaped workpiece into a shape including portions corresponding to the bottom portion (2, 3) and the left (4, 5) and right side wall portions such that the plurality of bend lines are formed at boundaries therebetween, and such that a flange portion is formed at an end of on the left side wall portion in the width direction, and a flange portion having hemming prongs is formed at an end of the right side wall portion in the width direction;; a second step of bending the workpiece, which has been formed in the first step, in such a direction that the portions corresponding to the left (4, 5) and right side wall portions approach each other by pressing a punch into a space between a pair of dies while clamping the portions corresponding to the bottom portion (2, 3) between the punch and a pad in a plate thickness direction; and a third step of bending the portions corresponding to the bottom portion (2, 3) and the left (4, 5) and right side wall portions along the bend-facilitating lines by pressing the portions corresponding to the bottom portion (2, 3) and the left (4, 5) and right side wall portions against an outer periphery of a plug having an outer peripheral shape that is the same as a final shape of the closed cross-sectional structure while the plug is placed on the portion of the workpiece corresponding to the bottom portion (2, 3), which has been formed in the second step; characterised that in the said first step, bend-facilitating lines are provided at the plurality of bend lines, wherein each of the bend-facilitating lines is a portion of the workpiece where a groove is formed in one surface thereof and a protrusion corresponding to the groove is formed on the other surface thereof, wherein a depth of the groove is greater than or equal to 0.05 times and less than or equal to 0.3 times a plate thickness of the workpiece and wherein a width of the groove is greater than or equal to 0.2 mm and less than or equal to 3.0 mm. press-forming the plate-shaped workpiece into a shape including portions corresponding to the bottom portion (2, 3) and the left (4, 5) and right side wall portions such that the plurality of bend lines are formed at boundaries therebetween, and such that a flange portion is formed at an end of on the left side wall portion in the width direction, and a flange portion having hemming prongs is formed at an end of the right side wall portion in the width direction;; a second step of bending the workpiece, which has been formed in the first step, in such a direction that the portions corresponding to the left (4, 5) and right side wall portions approach each other by pressing a punch into a space between a pair of dies while clamping the portions corresponding to the bottom portion (2, 3) between the punch and a pad in a plate thickness direction; and a third step of bending the portions corresponding to the bottom portion (2, 3) and the left (4, 5) and right side wall portions along the bend-facilitating lines by pressing the portions corresponding to the bottom portion (2, 3) and the left (4, 5) and right side wall portions against an outer periphery of a plug having an outer peripheral shape that is the same as a final shape of the closed cross-sectional structure while the plug is placed on the portion of the workpiece corresponding to the bottom portion (2, 3), which has been formed in the second step; characterised that in the said first step, bend-facilitating lines are provided at the plurality of bend lines, wherein each of the bend-facilitating lines is a portion of the workpiece where a groove is formed in one surface thereof and a protrusion corresponding to the groove is formed on the other surface thereof, wherein a depth of the groove is greater than or equal to 0.05 times and less than or equal to 0.3 times a plate thickness of the workpiece and wherein a width of the groove is greater than or equal to 0.2 mm and less than or equal to 3.0 mm.