We do not know of any modern cities free from air pollution. In Tokyo, the levels of nitrogen oxides, photochemical oxidants, and particulate matter often exceed the environmental quality standards. However, the cost and size of the chemical analyzers have limited the number of monitoring stations and, thus, the spatial resolution of the measured pollutant distributions. The authors have been proposing a gas distribution analyzing system (GASDAS). The use of gas sensors enables compact and inexpensive sensing systems, and leads to a significant increase in the density of monitoring sites. Various network technologies can be used to form a sensing network for efficient data collection. As a first step in the development of GASDAS, a measurement system for nitrogen dioxide and ozone has been developed using electrochemical and semiconductor gas sensors. Encouraging results have been obtained from both types of sensors, and the major interference gas to be addressed in future work has turned out to be nitrogen monoxide. A monitoring system for particulate matter has also been developed. A photosensor is used to measure the accumulation of particulate matter on an air filter. The experimental results have shown the potential of this simple method.