To obtain a 3D preform which may be folded into a fixed shape without distortion of fibers comprising the preform.

Fiber 12B' is woven to be short in area 30 and to be long in area 32. These differences in length are equalized when the preform is folded into the fixed shape so that the desired lengths are obtained. When the fiber is folded into the fixed shape, a tension is applied to the short portion and a drawing is caused in the long portion so that the fiber 12B' length on both sides is equal and correct in the folded portion and avoids the distortions and buckles.