groups include an isocyanate compound such as isophorone diisocyanate hexamethylene diisocyanate xylene diisocyanate hydrogenated xylene diisocyanate 4 4 39 diphenylmethane diisocyanate 4 4 39 methylenebis cyclohexyldiisocyanate adducts thereof oligomers thereof blocked diisocyanates prepolymers having isocyanate groups melamine resins such as an isobutylated melamine a n butylated melamine a butylated urea melamine resin and a methylated melamine epoxy resins and dialdehyde compounds etc in the curable resin composition of the present invention the mixing ratio of the component a a lactone modified polyvinyl acetal resin with the compound b having functional groups capable of reacting with hydroxyl groups ranges from 95 to 40 by weight preferably from 80 to 50 by weight in the case when it exceeds 95 by weight the resulting resin would become unnecessarily rigid and in contrast in the case when it is below 40 by weight the resulting resin would become unnecessarily flexible because of the increase in lactone chains and the solubility of the resin into various solvents would become poor because of the crystallinity of the lactone chains as mentioned hereinabove the lactone modified polyvinyl acetal resin of the present invention has primary hydroxyl groups at terminals of side chains which are higher in reactivity with isocyanate.