The data from several rural stations of the air quality monitoring network in Japan are useful for representation of the tropospheric ozone on the ground surface. The seasonal variation of the tropospheric ozone is investigated by using these data along with those from ozonesonde soundings at Sapporo, Tateno and Kagoshima. They show a regular pattern of a summer minimum as well as a spring maximum which is a common feature of the tropospheric ozone at middle latitudes in North America and Europe. The summer minimum implies that photochemical production is not a principal factor to control the tropospheric ozone, and it may be attributed to the weak transport of ozone from the stratos-phere, being associated with the fact that the tropical marine air mass prevails over Japan in summer.