Abstract: A prepreg is put on upper faces of a protruding jig and heaters arranged on and lifters. A vacuum bag covers upper portions of the protruding jig and the lifters together with the prepreg. The vacuum bag is fixed to flanges of a box-shaped jig. Then, a closed space including a space covered by the vacuum bag and a space communicated with the space covered by the vacuum bag is vacuumed. Then, the heaters are energized with electricity so as to heat and soften portions of the prepreg positioned on the lifters. After the portions have been softened, the lifters are lowered so as to fold and form the prepreg. Thereby, a product can be obtained in which no wrinkles are generated in folded portions.