PURPOSE: To reduce remarkably the retained foam in a prepreg by causing forcedly air in base material to flow out with resin varnish.

CONSTITUTION: While base material 1 is brought into contact with the porous base material-receiver 4 of semicylindrical shape and is moved, it is dipped into a resin varnish tank 2. With said movement, the resin varnish 3 is forcedly caused to flow and to pass through the porous base material-receiver 4 of semicylindrical shape, and then the varnish is deaerated and circulated, whereby the base material 1 is impregnated with varnish 3. Consequently, the retained foam in a prepreg can be reduced.