Provided are a circular polarization filter including a circularly-polarized light separating layer (preferably, a layer having a cholesteric liquid crystalline phase fixed therein or a laminate including a reflective linear polarizer and a λ/4 phase difference layer), in which the circularly-polarized light separating layer selectively transmits either right-handed circularly polarized light or left-handed circularly polarized light in a specific wavelength region, a transparent medium which is transparent with respect to light in the specific wavelength region is provided at least on one surface side of the circularly-polarized light separating layer, and the transparent medium has an inclined surface which forms an angle of 1° to 30° relative to the surface on the transparent medium side of the circularly-polarized light separating layer, and sensor system using the circular polarization filter. The circular polarization filter of the invention is capable of providing circularly polarized light with a high circular polarizance, or improving sensitivity in the sensor system using circularly polarized light.