PURPOSE: To prevent momentary discharge of pressure gas and reduce the quantity of remaining liquid at the time of completion by providing a resistant body having a longitudinal groove in the upper part of the discharge port of a reaction container of highly viscous substance.

CONSTITUTION: A resistant body 5 on which a longitudinal groove is cut is provided in the upper part of the discharge port 4 of a reaction container 1 that treats highly viscous substances such as synthetic polymer of polyester etc. When the quantity of remaining liquid in the container 1 is decreased, the molten polymer of the highly viscous substance flows out in a slitlike passage 6 to prevent spouting of pressure gas. Further, the quantity of remaining liquid at the time of completion of discharge can be reduced and productivity and work efficiency can be improved.