In a multilayered optical thin-film filter having multiple quarter
wavelength thick optical thin films with different refractive indices, the
filter includes a plurality of unit cavities that are stacked multiple
times via a connection layer, each unit cavity including a first reflector
layer, a spacer layer in contact with the first reflector layer, and a
second reflector layer in contact with the spacer layer. The first
reflector layer is a layered body and the second reflector layer is a
layered body. The spacer layer is a multilayered optical thin film with a
high refractive index obtained by layering even number of films, or is
composed of a multilayered optical thin film with a low refractive index.
The connection layer is a multilayered optical thin film with a low
refractive index obtained by layering odd number of films.