The oscillation wavelength of a variable wavelength laser is measured by using an optical comb, and a feedback control is performed, whereby laser beams of plural predetermined wavelengths are obtained, or a variable wavelength laser is caused to oscillate at plural arbitrary wavelengths to obtain plural distance measurement values. The wavelengths (frequencies) of laser beams when the respective distance measurement values are obtained are measured by the optical comb, and used in calculation of the geometric distance. Plural lasers are used, and the geometric distance is obtained while the wavelengths of laser beams oscillated from the lasers are measured by the optical comb.