The invention provides a method for preforming a prepreg material impregnated with thermosetting resin continuously in an uncured state. Sheets of prepreg material are pulled out from rollers and laminated, which is then bent to a predetermined cross-sectional shape in a bending device. The prepreg material is formed into a long product continuously in a mold having a heater. The temperature and pressure applied to the prepreg material in the mold is selected so that the heating temperature is either room temperature or a heating temperature low enough to prevent the thermosetting resin from complete curing, and so that the pressure is within a pressure range enabling the prepreg material to maintain its molded state.