PURPOSE: To enhance the separation efficiency of an objective isotope by efficiently exciting only the objective isotope by sweeping the frequency of irradiated laser beam in the vicinity of the optical resonance absorption line of the objective isotope to suppress the excitation ratio of the non-objective isotope.

CONSTITUTION: When the transition line 7 of an objective isotope having no ultrafine structure and a plurality of the transition lines 8-11 of a non-objective isotope having an ultrafine structure are present in an overlap frequency region, the frequency of laser beam having to irradiate is swept in the aforementioned frequency region. Since the objective isotope has one transition line 7, the excitation level density of the atom of the objective isotope is adiabatically subjected to reversal distribution. Since the non-objective isotope has an ultrafine structure, even when reversal distribution is generated through the principal transition line 8, the aforementioned reversal distribution is relaxed through collateral transition lines 9-11. Therefore, the separation efficiency of the objective isotope is enhanced.