Abstract: A novel silane coupling agent is proposed for the pretreatment of a glass fiber-based or mica flake-based inorganic reinforcing material prior to compounding with an epoxy resin or a polyimide resin as the matrix to give a composite material such as laminates having excellent heat resistance against molten solder alloy and resistance against heat shock. The principal ingredient of the treatment solution is a trialkoxysilylalkyl-substituted polymethylene diamine compound of the general formula (R.sup.1 O).sub.3 SiR.sup.2 NH(CH.sub.2).sub.n NHCH.sub.2 C.sub.6 H.sub.5, in which R.sup.1 is a methyl or ethyl group, R.sup.2 is a diavalent hydrocarbon group having 1 to 6 carbon atoms, e.g., methylene and propylene, and n is an integer of 4 to 8, or a hydrochloride thereof. This diamine compound is soluble in water, in particular, acidified with acetic acid and the aqueous solution is advantageously stable in storage as compared with conventional silane coupling agents.