A thermosetting resin composition which comprises (A) a resin in which at least one compound having a graft-reactive group such as a carboxyl group, a primary amino group or a secondary amino group and a carboxylic anhydride group has been grafted on a polycarbodiimide and (B) an epoxy compound. The said thermosetting resin composition retains heat resistance, electrically insulating properties and mechanical properties characteristic of polycarbodiimide, is excellent in solubility in various solvents and storage stability in the form of a solution and can be easily cured under mild conditions, and can produce a cured product excellent in humidity resistance, transparency, chemical stability and adhesiveness to various substrates.