To provide a knock-out drum with high gas-liquid separation efficiency of mixed vapor for an outer diameter of a drum body, which efficiently separates the mixed vapor into a liquid component and a gas component, while it is downsized in drum body.

The mixed vapor containing two or more components is made to flow from inflow pipes 2a, 2b into the drum body 1. An area difference between a cross-sectional area D2 of the inflow pipes 2a, 2b and a cross-sectional area D1 of the drum body 1 is utilized for liquefying a part of components of the mixed vapor. The knock-out drum 10 separate the mixed vapor into the liquid component and the gas component thereby. The knock-out drum 10 has the drum body 1 connected with two or more inflow pipes 2a, 2b.