PURPOSE: To make it possible to use the titled apparatus for a long period of time without generating clogging, by a method wherein a helical fin pivotably supported in a freely rotatable manner is provided in the apparatus and, while vapor and mist are coagulated and separated by said helical fin, they are returned to the lower part of the main body thereof to be collected.

CONSTITUTION: For example, a gas containing Na vapor and mist thereof is introduced into a container 1 from inlet nozzle 6 and passed through a helical fin 8 to be exhausted out of the container 1 from an outlet nozzle 7. Said helical fin 8 is attached to a rotary shaft 9 and rotated thereby. The vapor and the mist contained in the introduced gas is condensed to and trapped by the helical fin 8 and the inner wall of the container 1 and returned downwardly along the helical fin 8 in a liquid state. Because a temp. is low in the vicinity of the outlet nozzle 7, the return current is inhibited by surface tension, but because the helical fin 8 is rotated, said vapor and mist in the liquid state is forcedly returned to the downstream direction. The liquid recirculated to the downstream direction is detected about the liquid level thereof by a liquid level meter 19 and discharged by opening or closing a valve 24 according to the signal therefrom.