PURPOSE: To make it possible to obtain favorably impregnated prepreg by simple device by a method wherein fibrous base material is passed through a space, which is surrounded by a plurality of or concretely three or more rolls adjacent to one another and evacuated, and in succession, passed through an impregnating tank in order to be impregnated with varnish.

CONSTITUTION: Four rolls 1, 2, 3 and 4 rotate under the state being in close contact with one another. Further, the lower parts of the rolls 3 and 4 among the above-mentioned rolls are immersed in varnish 7 in a varnish impregnating tank 6. A space 5 is formed by being surrounded by four rolls. The space 5 is evacuated for better impregnation of base material with varnish. The enough evacuation of the space 5 is 100Torr or less and the preferable one is 60Torr or less. The sealing between the rolls is established by using elastic rolls, the surface of each of which is covered by elastic material such as rubber or the like. The side sealing is performed by cutting grooves 11 near the ends of each roll and fitting sheet-like bodies 12 equipped with packing 13 to the grooves 11 so as to seal one another.