An automatic system for seismo-geochemical observation has been improved to monitor small changes in subsurface gas composition. This system is computer-based and consists of an on-site monitoring unit for gas analyses and a telemetry unit for transmitting the observation data to our laboratory. The analytical data with this system showed three coseismic anomalies in spring gas compositions at a monitoring station for these 11 months. These anomalies were associated with a far (and large) earthquake or two small (and near) ones, respectively. The gas anomaly as a spike-like signal appears only for a short period and can be detected on the basis of temporally dense data provided by the new system. The result shows that the behavior of subsurface gases is sensitive to crustal movements. ¿American Geophysical Union 1991