There is provided an insulating polymeric-material composition that is superior in insulating performance and mechanical strength and exerts no adverse influence on the global environment even after discarded. This insulating polymeric-material composition is obtained by mixing lignin as a curing agent with an epoxidized linseed oil and then conducting a heating treatment to cure it. As the lignin, for example, one obtained by blasting a lignin raw material and then conducting an alcohol extraction is used. The epoxidized linseed oil and the lignin are mixed together in such a proportion that epoxy equivalent of the epoxidized linseed oil:hydroxyl equivalent of the lignin=1:1. In the composition, as a curing acceleration agent, for example, 2-ethyl-4-methylimidazole is added by 0.2-2.0 parts by weight relative to 100 parts by weight of the epoxidized linseed oil. Upon this, it is cured under a condition, for example, of a heating temperature of 150-170.degree. C. and a heating time of 10-20 hours. In some cases, the heating temperature is set to be formed of two different temperature regions.