A high-accuracy air interferometer comprising a laser beam source for emitting a laser beam, a first nonlinear optical crystal for receiving the laser beam from the laser beam source and converting a part thereof into a first second-harmonic wave, a reflecting mirror for reflecting the laser beam and the first second-harmonic wave, a conveyance means for conveying the reflecting mirror, a second nonlinear optical crystal for receiving the laser beam and the first second-harmonic wave reflected by the reflecting mirror and converting a part of the laser beam into a second second-harmonic wave, means for producing interference fringes by interference between the first second-harmonic wave produced by the first nonlinear optical crystal and the second second-harmonic wave produced by the second nonlinear optical crystal, and means for counting the interference fringes and dividing the number of interference fringes counted by a distance moved by the reflecting mirror.