PURPOSE: To prevent the occurrence of cracks and to improve the effect in reducing beat sounds by mixing a carboxylic acid represented by a specific structural formula, multivalent alcohol and a polymetric monopolymer.

CONSTITUTION: A carboxylic acid containing an unsaturated carboxylic acid represented by structural formulasIand II as a main ingredient is reacted with multivalent alcohol to obtain an ester oligomer. This ester olygomer of 30W 80wt% is mixed with a polymetric monopolymer of 70W20wt% which is capable of copolymerizing with an ester oligomer, so as to attain a hardening resin composition used for processing a transformer. This composition has good adhesion property with component materials of the transformer and can prevent occurrence of cracks. In the formulas (n) represents two or three and (X) is an α,β- unsaturated hydrocarbon group with the number of carbons being two or three.