Coseismic changes in the radon concentration of groundwater were consistently observed at a sensitive observation station for all nearby earthquakes (100--130 km away) with magnitudes of 6.0 and over, and for some earthquakes of the same magnitudes occurring up to about 400 km away. A total of 11 coseismic changes, larger than 2&sgr; of the noise level, were observed for 16 earthquakes which occurred during the observation period, between January 1984 and July 1988. A similar change was also found for an M7.9 earthquake at a distance of about 1000 km. The radon changes lasted a few days in most cases; they may be caused by changes in the earth's stress field.