An interferometer comprising: a measurement optical system in which measurement light is split by a beam splitter into light beams, which are guided to a movable mirror and to a stationary mirror, respectively, and the light beam reflected off the movable mirror and the light beam reflected off the stationary mirror are combined by the beam splitter into measurement interference light, which is guided to a measurement light photodetector; and a reference optical system in which reference light from a reference light source is split by the beam splitter into light beams, which are guided to the movable mirror and to the stationary mirror, respectively, and the light beam reflected off the movable mirror and the light beam reflected off the stationary mirror are combined by the beam splitter into reference interference light, which is guided to a reference light photodetector, the interferometer performing measurement of the measurement interference light while, based on a detection result of the reference light photodetector, detecting a position of the movable mirror, characterized in that the reference light source is constituted by one or other of a semiconductor laser and a semiconductor laser optically coupled to a waveguide or a fiber, the reference optical system has a collimator optical system for reference light that converts laser light emitted from the reference light source into collimated light, and the collimated light becomes obliquely incident on a same planar surface of the stationary mirror as the measurement light, such that the optical path of measurement light and the optical path of reference light are not perfectly coaxial with each other and even if reference light reflected off the stationary mirror returns toward the reference light source, incidence of the returning light on the reference light source is prevented. a measurement optical system in which measurement light is split by a beam splitter into light beams, which are guided to a movable mirror and to a stationary mirror, respectively, and the light beam reflected off the movable mirror and the light beam reflected off the stationary mirror are combined by the beam splitter into measurement interference light, which is guided to a measurement light photodetector; and a reference optical system in which reference light from a reference light source is split by the beam splitter into light beams, which are guided to the movable mirror and to the stationary mirror, respectively, and the light beam reflected off the movable mirror and the light beam reflected off the stationary mirror are combined by the beam splitter into reference interference light, which is guided to a reference light photodetector, the interferometer performing measurement of the measurement interference light while, based on a detection result of the reference light photodetector, detecting a position of the movable mirror, characterized in that the reference light source is constituted by one or other of a semiconductor laser and a semiconductor laser optically coupled to a waveguide or a fiber, the reference optical system has a collimator optical system for reference light that converts laser light emitted from the reference light source into collimated light, and the collimated light becomes obliquely incident on a same planar surface of the stationary mirror as the measurement light, such that the optical path of measurement light and the optical path of reference light are not perfectly coaxial with each other and even if reference light reflected off the stationary mirror returns toward the reference light source, incidence of the returning light on the reference light source is prevented.