To extend the kind of shape to be molded (1), to reduce the weight of a mold for molding a cylinder made of a glass fiber-reinforced plastic material and a hollow object which has no hollow on the outer surface and has a three-dimensional free surface to facilitate the transportation and handling of the mold (2), to change the material of the mold to make the mold foldable to save a space for storing the mold (3), to change an apparatus to shorten a molding work time (4), and to enable the integral molding of the whole surface (5) with regard to molding work by a filament winding method.

A conventional mold of wood or a hard synthetic resin is changed to a bag mold using a sheet high in dimensional stability and airtightness, and a mounting device is installed.