PURPOSE: To permit the easy manufacture of an ND filter substrate and fine adjustment of a film thickness after the formation of a thin film by using tantalum nitride as the substrate material for the ND filter and controlling the time for electricity conduction in the process for modifying the tantalum nitride by an anodic oxidation method to tantalum pentaoxide.

CONSTITUTION: After a glass substrate 3 is cleaned, a thin tantalum nitride film having the required film thickness is formed by a vacuum deposition method or sputtering method. Gold electrodes 1 are then formed on the respective tantalum nitride films 2. The substrate 3 is taken out of the sputtering device and is subjected to a thermal oxidation treatment in an oxygen atmosphere; thereafter the film thickness is finely adjusted. The fine adjustment of the film thickness is executed by forming a dike 7 in the boundary part between the electrodes 1 and the films 2 and the inside of the dike 7 is filled with an electrolyte 5. The films 2 are modified to the tantalum pentaoxide having an electrical insulating characteristic when a voltage is impressed between the tantalum cathode 6 and the anode of the electrodes 1. The thickness 4 of the formed tantalum pentaoxide film is controlled by the time for electricity conduction, by which the optical film thickness of the tantalum nitride is finely adjusted.