PURPOSE: To automatically control the wavelength of laser beam so that the center frequency of a laser spectrum synchronizes with that of an absorption spectrum by detecting the intensity of the fluorescence emitted from the isotope to be separated which in irradiated with laser beam.

CONSTITUTION: The first fluorescence detector 12 detects fluorescence when a specific isotope whose velocity vector is zero with respect to the advance direction of laser beam traverses the laser beam. The second fluorescent detectors 11, 13 detect at least either the fluorescence emitted when a specific isotope having a positive velocity vector with respect to the advance direction of laser beam traverses the laser beam or the fluorescence emitted when a specific isotope having a negative velocity vector. Further, a wavelength shift judging means 15 judges the shift direction of the absorption spectrum of a spepific isotope on the basis of the fluorescent detection signals of the detectors 11-13. A laser wavelength control part 16 controls the wavelength of laser beam on the basis of the judgment result of the means 15.