PURPOSE:To provide a thermosetting compsn. which gives cured articles having excellent heat resistance, adhesion and electrical insulating properties and is suitable for use as varnish, by mixing a specified polyparabanic acid with an epoxy resin having epoxy groups and a hardener. CONSTITUTION:A polyparabanic acid (A) composed of a repeating unit of formula I is mixed with an epoxy resin (B) having epoxy groups and a hardener (C) (e.g., dicyandiamide) to obtain a desired thermosetting compsn. The compsn. is suitable for use as varnish for use in the field of electronic components such as printed circuit boards, etc. The polyparabanic acid of formula I can be obtd. by reacting 4,4'-diphenylmethane diisocyanate with hydrogen cyanide and hydrolyzing the resulting polymer having an imidazolidinedione-1,3-diyl ring, composed of a repeating unit of formula II.