PURPOSE: To enable to simply demold a braided product without damaging the braided product by braiding fibers on a balloon-like mandrel in which a liquid is injected.

CONSTITUTION: Fibers or filaments are wound or crossed on the outer periphery of a parent mold capable of being disassembled into plural pieces and having a Y-shape, etc., so as to cover the parent mold, immersed in a polyurethane resin solution, etc., and, if necessary, subsequently thermally treated. The parent mold is disassembled to form a balloon-like mandrel M comprising a rubber film reinforced with the fibers or filaments. The opening end of the formed mandrel M is fit onto a cylindrical member 9 having a valve 8 therein through a clamping member 10, and a liquid such as an oil is injected into the mandrel M through the cylindrical member 9. The valve 8 is closed to obtain the mandrel M swollen in a specific shape. The mandrel M is mounted on the holding device of a braider, subjected to the formation of a braided product, and subsequently subjected to the impregnation and curing of a thermosetting resin, etc. Then, the valve 8 is opened to shrink the balloon-like mandrel M, and the shrunk mandrel M is simply taken out from the resincured braided product.