To provide a method for bending a closed cross section member capable of easily being implemented and capable of making a good appearance tube after bending.

After a slit 12 or window holes 13, 14, 15 are continuously shaped on a closed cross section member, for example, predetermined surfaces for bending (vertical surfaces) 11a, 11b of a rectangular tube 11 and right-angled surfaces (side surfaces) 11c, 11d and then, with respect to the method for bending the closed cross section member to bend the predetermined surfaces for bending 11a, 11b, a fine showing closed bending which causes no cross section deformation without using cores is easily executed. The slit width or the size of window holes to be opened on the predetermined surface of the bending and the right- angled surface can be changed so that the cross section dimension and the shape of the closed cross section member after bending can be changed to correspond with a variety of products. Further, when the slit or the window hole is shaped, bending is more easily executed and the bending die or jig requires no complicated construction, which enables less investment.