A dehumidifier is connected to a respiratory gas measuring apparatus. A membrane dryer has a first flow path through which a respiratory gas of a subject passes, and a second flow path through which a purge gas passes. A first normally-closed electromagnetic valve is disposed in a first upstream flow path that communicates with the first flow path. A second normally-closed electromagnetic valve is disposed in a first downstream flow path that communicates with the first flow path. A third normally-closed electromagnetic valve is disposed in a second upstream flow path that communicates with the second flow path. A fourth normally-closed electromagnetic valve is disposed in a second downstream flow path that communicates with the second flow path.