PURPOSE: To provide the method for sepn. of chemical species which enables baking in order to maintain vacuum, has no possibility of corrosion and does not require the exchange of parts due to corrosion.

CONSTITUTION: A device for forming and controlling another molecular beam in order to cut a molecular beam is installed in place of a device entailing mechanical driving in the vacuum where the molecular beam consisting of the plural chemical species is cut. Gas consisting of the plural chemical species is put into a container 1 and if the material consisting of the plural chemical species is originally solid or liquid, the material is previously made into gas by thermal excitation, etc. The container 1 is connected through a small hole 2 to a vacuum vessel 3. The gas put into the container 1 chokes at the hole 2 and thermally expands in the vacuum vessel 3 if the pressure in the vacuum vessel 3 is kept lower than the pressure in the container 1 to some extent. The gas under thermal expansion is collimated by a spacing 4 and forms the supersonic molecular beam 5.