PURPOSE: To obtain the titled resin composition having improved resistance to heat softening, by reacting a polycarboxylic acid containing citric acid with a diisocyanate and a lactam.

CONSTITUTION: A resin composition obtained by reacting a polycarboxylic acid containing 10mol% or more citric acid and/or a derivative thereof, preferably an aromatic tricarboxylic acid, particularly trimellitic acid, with a diisocyanate and/or a derivative thereof, preferably an aromatic diisocyanate and/or the aromatic diisocyanate masked with a phenol, particularly 4,4'-diphenylmethane diisocyanate, and a lactam, e.g. -caprolactam, in a phenolic solvent, and adding 5W150pts.wt., expressed in terms of polyisocyanate based on 100pts.wt. resultant resin content in the reaction product, polyisocyanate compound masked with a phenol to the reaction product.