A fabricating method of a thin film transistor substrate according to the present invention includes the steps of forming on a substrate material a thin film transistor array including a plurality of signal lines; forming an organic insulating film on the substrate material on which the thin film transistor array has formed; patterning the organic insulating film; performing a surface treatment on a surface of the organic insulating film using helium plasma; and forming a transparent electrode layer on the organic insulating film.