To achieve accurate measurement by inhibiting interference by the impurity concentration of an oxidizer even if the concentration of a measurement target in a sample is low.

Since an oxidation reaction section 12 has an ultraviolet lamp inside, an organic matter in a sample that is injected into the oxidation reaction section 12 is oxidized by the operation of the oxidizer and the ultraviolet rays and is converted to carbon dioxide. The carbon dioxide thus generated is carried to an infrared gas analysis section 42 by a carrier gas that is circulated to the oxidation reaction section 12 to measure concentration. A control section 8 first injects the oxidizer into the oxidation reaction section 12, performs the oxidation decomposition of organic matters contained in the oxidizer due to an oxidation reaction for specific time for removal, and then injects the sample to the oxidation reaction section 12 and starts detecting a peak by the infrared gas analysis section 42.