PURPOSE: To make it possible to remove an interfering component effectively by filling a container with gallium alloy in liquid state at ordinary temperature under the supported state on a carrier, or independently, and providing the container at the front stage of a CO2 detecting part.

CONSTITUTION: A container 40 is provided in a flow path for guiding gas (a) into a nondispersive infrared-ray type gas analyzer (NDIR). The container 40 is filled with glass beads 42, on which gallium alloy is supported. The sample containing high-concentration hydrochloric acid and inorganic salt is gassified by the reaction of thermal decomposition in a TC combustion tube. Interfering components such as chlorine and hydrogen chloride are contained in the gas and mist generated by the combustion. The interfering components react with the gallium alloy when the interfering components pass through a removing device 28, and the interfering components are removed.