A device includes a gas cell (10X) configured to form an introduction space (11X) into which a target gas is introduced, an infrared light source (20X) disposed at one end of the gas cell (10X), a modulation mirror (70X) disposed at one end of the gas cell (10X) and configured to reflect or transmit light emitted from the infrared light source (20X), a reflecting mirror (60X) configured to reflect light transmitted through the modulation mirror (70X), a saturated gas chamber (40X), in which a predetermined comparison gas is hermetically enclosed, disposed on an optical path of light transmitted through the modulation mirror (70X), a light receiving unit (30X) disposed at the other end of the gas cell (10X) and configured to receive light reflected by the modulation mirror (70X) and light transmitted through the modulation mirror (70X) and reflected by the reflecting mirror (60X) through the saturated gas chamber (40X), and a calculation circuit (3X) configured to calculate the concentration of the target gas based on received light energy values of the light receiving unit (30X) in each case in which light is reflected or transmitted by the modulation mirror (70X).