PURPOSE: To analyze an NOX continuously without any hindrance by sampling a gas exhausted from a thermal power plant, etc. from a flue, preparing the gas to the specified temp. range, and reacting NH3 and SO3 in the gas to remove the obstacles in measurement of the NOX in the gas.

CONSTITUTION: A sampling pipe 1 having a gas taking-in hole 2 at the flue 3 is fixed airtightly through an O-ring 7 by using an attaching pipe 4 and a fixing member 5. The pipe 1 is wound with a coil-shaped electric heater 9, the internal temp. of the pipe 1 is detected by a thermocouple thermometer 10 to control the heater 9 to 150W250°C range. Thus, NH3 and SO3 in the waste gas are changed to (NH4)2SO4 or NH4HSO4, removed together with the other dust by a wire net filter 14 provided at the front end of a conduit 11 in the pipe 1, the water and the remaining SO3 in the waste gas are removed by an electron cooler 19 from the pipe 11 through a valve 16, and the remaining gas containing NOX is sent to a detector from a conduit 21 through a mist remover 22. Thus, the continuous analysis of NOX is possible.