An object of this invention is to diminish a ratio of a concentration of N2O in the dilution air to a concentration of N2O in the measurement object gas diluted by the dilution air as much as possible by removing N2O in a dilution air so that a measurement accuracy of a concentration of N2O in a measurement object gas can be improved. A heater 33 that applies heat to the dilution air, a Pd catalyst 341 and a Pt catalyst 342 are arranged in this order on a flow channel where a dilution air used for diluting the measurement object gas flows, and N2O in the dilution air is oxidized to NOx or reduced to N2 by the Pd catalyst 341 and the Pt catalyst 342.