An interferometer of the present invention includes a PBS2 which splits light into reference light and measurement light, a reference mirror 4a which reflects the reference light entering the reference mirror from a first direction, a measurement mirror 4b which reflects the measurement light entering the measurement mirror from a second direction, a lens system 6 which reflected lights from the reference mirror 4a and the measurement light 4b enter, a reflective device 5 which reflects light from the lens system 6, and a light receiving device 16 which receives multiplexed light, wherein the reference mirror 4a and the measurement mirror 4b are in a conjugate relation with respect to the reflective device 5, and at least one of the reference mirror 4a and the measurement mirror 4b is tilted so that its normal direction differs from the first and the second direction.