PURPOSE: To purge a whole system and to accurately measure an exhaust gas by removing HC remaining in an exhaust gas-introducing path at every measur ing time.

CONSTITUTION: All of the pickup bags 22-27 are filled with air. A purge control device 33 permits a common pipe 28 to communicate with an exhaust gas introducing pipe 30, for example one bag 22 is connected to an analyzing device 32. At that time, the passing of the air in the bag 22 through the pipe 30 allows residual HC component to be purged and discharged out. The device 32 measures ' HC concentration in the air introduced through the pipe 30 and transfers the data to the device 33. The device 33 judges the transferred HC concentration in the pipe 30. When the concentration is greater than a judging- reference level, the purge treatment in the pipe 30 is repeated until the concentration is smaller than the level. As a result, the inner section of the pipe 30 becomes a condition that the next measuring of the exhaust gas is not influenced so that the next measuring is accurately executed.