It is an object to provide a novel forming agent for a gate insulating film that not only provides high insulating properties for the gate insulating film but also takes account of the electric characteristics of a thin film transistor element. A forming agent for a gate insulating film of a thin film transistor characterized by comprising an oligomer compound or a polymer compound including a structural unit containing a pyrimidinetrione ring having a hydroxyalkyl-containing group as a substituent on a nitrogen atom; a gate insulating film formed by the forming agent; and a thin film transistor.