A thin film circuit substrate is manufactured by forming a lower thin film electrode on a substrate, forming an organic insulating film with via holes on the lower thin film electrode, and irradiating the substrate with an inert gas ion to remove an oxidized surface film on the lower thin film electrode, and to generate functional groups, such that a modified surface layer with a surface modification coefficient of about 0.1 to about 0.5 is formed on the surface of the organic insulating film, and such that the oxidized surface film on the lower thin film electrode is removed at the same time. Accordingly, a thin film circuit substrate having excellent adhesion strength between the organic insulating film and the upper thin film electrode as well as excellent reliability of electroconductivity between the upper and the lower thin film electrodes is efficiently manufactured.