The NDIR (nondispersive infrared) monitor provided with a smart network interface is developed for distributed measurements of CO2 gas concentration in the atmospheric air. Two thermopiles in a single package, one for measuring the spectrum absorption at 4.26 μm band due to CO2 molecules in the air and the other for monitoring the infrared (IR) intensity emitted by an incandescent lamp, are used for canceling the effects of temperature and aging by the ratiometric signal processing. The double sampling is also used for canceling the offsets of thermopiles and the signal conditioner. These signal processing techniques and the online calibration make it possible to achieve the measurement accuracy of ±40 ppm over the CO2 concentration range from 500 to 2000 ppm with low-cost implementation. The temperature dependence is 1.5 ppm/°C, and the response time is 1.5 min. Access to the monitor through the Internet is made by a Java applet and client programs in the C language.