PURPOSE: To efficiently separate and recover the desired ion by using a method for controlling the ion density and recovering the ion in stages and selecting the number of stages, intensity of a laser beam and cycle period of irradiation even when the vapor density is high.

CONSTITUTION: An ion recovery electrode A is arranged along the traveling course of a vapor flow C contg. ≥2 kinds of isotopes, and the flow C is irradiated with a laser beam for selectively ionize a specified isotope. The formed ion of the specified isotope is electrostatically adsorbed on the electrode A, and the specified isotope is separated. The part of the traveling course along the electrode A is divided into plural regions E1-E5 starting from the upstream side. Whenever a vapor flow block F defined by the size of the sectional region passes through the sectional region, the sectional region is irradiated with a laser beam having a cross-sectional area capable of practically covering the sectional region and capable of ionizing the specified isotope in the block F in a specified ratio.