Abstract: A channel material 2 of predetermined length is fed into a stretch forming apparatus 1 and bending the material 2 in predetermined form. A mold 3 in the apparatus 1 is placed in symmetry forward and rearward of the apparatus 1 so as to bend the channel material in lateral symmetry, arranging a plurality of cores 4 along the mold 3 in such a manner that they can be inserted into and removed from the inside of the material 2 to be fed into the apparatus 1. The arranged cores 4 are supporting in such a manner that they can slide forward and rearward along the bend part of the material 2, and the material 2 is bent in predetermined forward and rearward directions while stretching both end portions of the material 2 by means of stretch cylinders 13 so as to form a forward and rearward symmetric molding. Thus, it is possible to stretch-form at high production efficiency even a laterally symmetric channel material for a vehicle frame or the like.