PURPOSE: To prevent a fray of the side end part of a weft and thickening of the side end part, by a method wherein an end part of the weft of glass cloth is sealed with thermoplastic resin along the warp.

CONSTITUTION: Sealing 3 is performed on an end part of the weft 2 on both side end parts of a widthwise direction of glass cloth A. To perform the sealing 3, either varnish of thermosetting resin is dried and cured by coating the same or a tape of thermosetting resin is hot-press-bonded. To make prepreg by making use of the glass cloth A, the long-sized glass cloth is passed through an impregnation tank into which thermosetting resin varnish such as epoxy resin is fed while drawing out continuously the rolled long-sized glass cloth and the glass cloth is impregnated with the thermosetting resin varnish. Then the prepreg is obtained by drying the glass cloth by heating the same by passing through a drying furnace.