The light intensity ratio adjustment filter is placed between the reference surface and the sample surface of the interferometer. This light intensity ratio adjustment filter has a light intensity ratio adjustment film including an optical reflection-absorption layer and a dielectric anti-reflection layer on the surface of a transparent substrate made of glass on the sample side, and an optical anti-reflection film on the reference surface side, and acts so as to reflect part of the incident light from the surface opposite the reference surface, and after absorbing part of the remaining light, transmit the remainder towards the sample, and furthermore, absorb part of the light returned from the sample while controlling reflection, and transmit the remainder in the direction of the reference surface as the sample light.