PURPOSE: To eliminate the need for temperature control over a sensor part and to simplify the constitution of the sensor part by using a long-wavelength infrared-ray semiconductor laser which oscillates pulses with specific wavelength at room temperature as an infrared-light source.

CONSTITUTION: The long-wavelength infrared-ray semiconductor laser 12 is a semiconductor laser which oscillates with almost 4.3μm wavelength and held at constant temperature by temperature control means 13, 14, and 15. Then pulse modulation is imposed by a pulse driving means 17 and the pulses are projected. The projected laser light is made incident on a converging optical element 16 and converted into converged luminous flux, which is made incident on a pyroelectric detecting element 7 through a gas cell 4. Consequently, the element 7 outputs an electric signal which is proportional to the quantity of transmitted light and the signal is converted into the concentration of carbon dioxide gas. Therefore, the sensor part 11 needs not be brought under strict temperature control, the constitution can be simplified, and the cost is reducible.