PURPOSE: To prevent the deterioration in the band characteristic of transmission wavelength and the decrease in trasmitted light and to a deal with the diversity in the transmission wavelength band by using a dielectric material having the intermediate refractive index of the refractive indexes of the thin dielectric films to be laminated alternately for a cavity layer.

CONSTITUTION: Titainum dioxide having 2.25 refractive index is deposited as a thin film having 217.5nm optical film thickness by vacuum evaporation on a glass substrate heated to 300°C as shown in the table and magnesium fluoride having 1.38 refractive index is deposited to the same optical thickness thereon by vacuum evaporation. The thin dielectric films consisting of the titanium dioxide and magnesium fluoride are alternately and repeatedly formed into 11 layers. Cerium fluorine having the refractive index between 2.25 and 1.38 (refractive index = 1.58) is deposited by evaporation as 435nm thin film if the order of the film formation is the intermediate 12th cavity layer. The filter which is free from the deterioration in the transmission band characteristic and the decrease in the transmitted light and has the excellent spectral characteristic is obtd. if 23 layers of the thin dielectric films are formed in lamination on said cavity layer.