PURPOSE: To make continuous analysis feasible by a method wherein two gas suction pipes are inserted into a flue and along with this communicating tubes, intermediate communicating tubes and cross valves are arranged.

CONSTITUTION: Along with two gas suction pipes 1, 1' inserted into flue 7, communicating tubes 7,7', intermediate communicating tube 8 and a pair of cross valves 9, 9' are arranged. And therewith, when a stream from gas suction pipe 1 through communicating tube 7', intermediate communicating tube 8, communicating tube 7 to gas suction pipe 1' by properly operating a pair of cross valves 9, 9' and suction pump 10 is operated, the exhaust gas is extracted from flue 7 via primary filter 2 and exhausted again into flue 7, and when pump 6a is operated, the exhaust gas passing through secondary filter 5 is sucked into analyzing meter 6. On the other hand, when a pair of cross valves 9, 9' are changed-over, the exhaust gas extracted from gas suction pipe 1' is exhausted again into flue 7 via primary filter 2 and primary filter 2 is riused in reverse direction to achieve the expected purpose.