PURPOSE: To readily acquire prepreg of good defoaming property at a low cost regardless of a kind of silane coupling agent by treating glass cloth by using ammonium chloride in addition to the silane coupling agent.

CONSTITUTION: A first, 0.5wt.% water solution of N--(aminoethyl)-γ- aminopropyl trimethoxysilane hydrochloride is prepared, and 0.2wt.% of ammonium chloride and acetic acid are added thereto to make PH 4.0. Glass cloth WEA 18W105 is immersed in the treatment liquid, squeezed by a mangle to realize treatment liquid keeping ratio of about 30% to glass cloth, and dried for 5 min at 110°C. Then, resin varnish is prepared according to NEMA standard FR-4 prescription. The glass cloth is impregnated with the prepared varnish, and dried at 130°C for 7 minutes to form a prepreg. Eight sheets of the prepreg are laminated, copper foil is further laminated up and down thereof and they are heated and molded at a pressure of 30kg/cm2 at 170°C for 90min to form a copper clad laminated board. Copper foil is removed from the laminated board by etching.