PURPOSE: To prevent the local clogging of a packing material and to prolong the life of a mist trap, by impinging a high temp. Ar gas containing Na mist vapor to a condensing plate cooled by a low temp. Ar gas within the mist trap.

CONSTITUTION: A high temp. Ar gas containing Na mist vapor is flowed into a mist trap from an Ar gas inlet nozzle 4 and impinged to a condensing plate 8 cooled by an Ar gas flowing in from a low temp. Ar gas inlet nozzle 7. The Na mist vapor is rapidly cooled and adhered to the vertical surface of the condensing plate 8 in Na liquid droplets to be flowed down to a drain nozzle 9 provided to the lower part thereof along the condensing plate 8. On the other hand, a greater part of the Na mist vapor is removed at the part of the condensing plate 8 and the Ar gas reduced in the concn. thereof is flowed into a packing material 2 provided to the upper part thereof and flowed out from an Ar gas outlet nozzle 5 after the residual Na mist vapor is removed by the packing material 2.