PURPOSE: To separate an isotope mixture by selectively changing the diffusion coefficient or transmission coefficient of said mixture in a molecule and ion separation membrane, by vibrating and exciting a molecule constituting the separation membrane by laser beam.

CONSTITUTION: An apparatus main body 1 is partitioned into two chambers 1a, 1b by a separation membrane 2 comprising BCl3 and a mixture to be separated is introduced into the chamber 1a. In this case, said mixture is supplied from an inlet pipe 1c and appropriately discharged from an outlet pipe 1d. On the other hand, a laser irradiating window 3 is formed to the part opposed to the separation membrane 2 of the chamber 1b and laser beam 4 from CO2 laser is irradiated through the window 3 to vibrate and excite BCl3. In addition, a recovery pipe 5 for a separated substance is connected to the side wall of the chamber 1b and connected to the inlet side of a trap apparatus 8 through an opening and closing valve 6.