To prevent a product from being affected by a breakthrough and to improve use efficiency by using active carbon to a limit by break through when chlorides in exhaust gas discharged from a CVD reactor used in the production of polycrystalline silicon are adsorbed/removed by active carbon.

In a method, a small-size subordinate adsorption container 20 filled with a small amount of active carbon is arranged downstream from a main adsorption container 10 filled with a necessary amount of active carbon. In piping 30 connecting the container 10 with the container 20, the break through of the container 10 is detected from the amount of chlorides in gas. Even when the container 10 is broken through, contaminants are adsorbed/removed by the container 20.