To prevent the generation of cut powder upon cutting, and to prevent the generation of glass fiber powder upon treatment after cutting.

Each prepreg P obtained by impregnating a glass fiber base material 2 with a thermosetting resin and subjecting the same to half-setting is irradiated with laser beams 6, 9, and the prepreg P is cut. Also, by the setting of the thermosetting resin after the cutting, the glass fiber base material 2 is fixed in a state where the cut edge side thereof is wrapped with the thermosetting resin. At this time, the prepreg P is cut as the laser beams 6, 9 are relatively moved in the cutting direction to the prepreg P, and, to the prepreg P, jetting regions including irradiating regions of the laser beams 6, 9 are jetted with a gas for cooling, and the cut places of the prepreg P are cooled by the gas for cooling.