There is provided a novel gate insulating film forming material in consideration of not only initial electric properties immediately after the production of a gate insulating film, but also electric properties after other steps are performed while producing a thin-film transistor using the gate insulating film, and even reliability in the electric properties of the produced element. A gate insulating film forming agent for a thin-film transistor comprising an oligomer compound or a polymer compound, both of which contain a repeating unit having a triazinetrione ring containing a hydroxyalkyl-containing group as a substituent on a nitrogen atom, and a solvent; a gate insulating film produced from the gate insulating film forming agent; a thin-film transistor having the gate insulating film; and a method for producing the gate insulating film or thin-film transistor.