To easily and highly precisely form a film even in an optical multilayered film filter with high difficulty of film formation.

In the method of forming films of an optical multilayered film filter by using an optical film thickness meter (OTM) 15 to detect the film thickness and controlling the film forming device 11 based on the detection output, the light source of the OTM 15 is a wavelength variable light source 12 which can vary the wavelength in the range from λ1 nm to λ2 nm including the wavelength λ nm. The optical film thickness of each layer designed as a λ/4 structure is optimized to the range from λ1/4 nm to λ2/4 nm, the wavelength of the wavelength variable light source 12 is selected to give the maximum transmittance in the optical film thickness of each layer, and the extreme of the transmittance is detected to control to complete the film formation of each layer.