There is provided a novel composition for forming a gate insulating film taking into consideration also electrical characteristics after other processes such as wiring by irradiation with an ultraviolet ray and the like during the production of an organic transistor using a gate insulating film. A composition for forming a gate insulating film for a thin-film transistor comprising: a component (i): an oligomer compound or a polymer compound containing a repeating unit having a structure in which a nitrogen atom of a triazine-trione ring is bonded to a nitrogen atom of another triazine-trione ring through a hydroxyalkylene group; and a component (ii): a compound having two or more blocked isocyanate groups in one molecule thereof.