Method for curing resin

In a method for curing a resin by utilizing an intermolecular cross-linking reaction, the improvement wherein the cross-linking reaction is initiated by the ring opening of a lactone ring and addition reaction with an active hydrogen-containing functional group involving

• a mixture of (A-1) a resin containing at least two 4- to 9-membered lactone rings per molecule and (A-2) a second resin containing at least two active hydrogen-containing functional groups selected from the group consisting of a hydroxyl group, a carboxyl group and an amino group and having a number average molecular weight of 1,000 to 50,000;
• said lactone rings each being bound to a main chain of the resin as pendant and/or terminal groups.