To obtain a solventless type thermosetting composition, shrinking uniformly and to a limited extent during the setting process, and capable of forming a coating film excellent in surface smoothness and also of forming a non-adhesive unset coating film, and also to provide a thermally set coating film, film-provided solventless type thermosetting composition and interlayer coating film for build-up circuit substrates using the above composition.

This solventless type thermosetting composition contains, as the essential ingredients, (a) 20 to 95 pts.wt. of a powdered resin having a particle size of 50 μm or less and melting point of 30°C or higher, and (b) 80 to 5 pts.wt. of a liquid at normal temperature, self-curable and having a thermosetting group capable of crosslinking under heating, or containing a curable group capable of crosslinking by reacting with the functional group in the powdered resin component (a). This method forms a thermally set coating film by thermally setting the above composition under specific conditions.