PURPOSE: To eliminate the flowing out of a liq. to the outside of the apparatus and to improve extremely the safety of the operation by providing a liq. stopper having volume larger than the total volume of a liq. to be stored in the fluid treating apparatus on the upstream part.

CONSTITUTION: A gas is introduced into the inside of an apparatus from the upstream part 1, passed through a gas stopper 5, injected into a liq. 4 from a blow-off part 3, passed through the liq. 4, and discharged from the downstream part 2 to the outside of the apparatus. When the pressure at the downstream part 2 is increased and the liq. 4 is refluxed, the liq. 4 is received in a liq. stopper 5 and is not discharged to the outside of the apparatus. Moreover, the liq. in the liq. stopper 5 flow out from the blow-off part 3 when the pressure on the upstream part side is increased, and returns to the bottom of the apparatus. Accordingly, the gas can be sent again into the liq. In this case, when the bottom part 11 of the liq. stopper is inclined at a specified angle to the blow-off part 3 side, the liq. in the liq. stopper 5 can be easily returned to the bottom of the apparatus.