Long-term (1970–2002) ozonesonde data from four sites in Japan (Sapporo, Tsukuba, Kagoshima, and Naha) have been analyzed using a sectoral air mass classification based on back air trajectories. Air masses from “regionally polluted,” “Eurasia,” Pacific, and Japan regions are tagged with daily ozone values at each of the four sites. Ozone in regionally polluted air masses shows a broad maximum in spring and summer and a minimum in winter, whereas ozone in other air masses shows a maximum in spring and minimum in summer at Tsukuba, Kagoshima, and Naha. The wintertime ozone in regionally polluted air masses is highest but summertime ozone is lowest at Naha among all other sites. In the lower troposphere the air masses from southern China also show much lower ozone levels in summer than from the northern part (up to 36 ppbv in August), which is suggested to be due to the additional influence of an Asian monsoon over southern China. Ozone levels at Sapporo are determined mainly by Eurasian air masses and show a broad maximum in spring and summer. The boundary layer ozone levels in the regionally polluted air masses are 10–12 ppbv higher throughout the year than in the Eurasian air masses, and in winter they are about 8 ppbv higher than over Europe. However, ozone levels and seasonal variations are similar over Japan and Europe in the lower troposphere. Ozone levels in the regionally polluted air masses increased by 11–20% (5–11 ppbv) from 1970s to 1990s over Japan, and this is attributed to the large increase in NO_x emissions over China in the 1990s. The long-term trend (between 1970 and 1990) in regionally polluted ozone is observed not to be very different over Japan and Europe, particularly in the lower troposphere. However, during the 1990s the wintertime positive trend (0.2–0.4 ppbv yr⁻¹) in lower tropospheric ozone in the Eurasian air masses at Sapporo is suggested to be due to the intercontinental transport from Europe/North America. However, the inconsistency between the long-term trend in the regionally polluted ozone and the NO_x emissions over China in the 1990s remains unexplained.