Since August 1992, tropospheric ozone has been measured at the top of Mt. Fuji to investigate the background ozone feature of middle troposphere over Japan. During one year, the maximum concentration of monthly mean ozone at Mt. Fuji was 59.3 ppbv in may and minimum was 30.9 ppbv in August. The secondary minimum was recorded in January. The seasonal behavior showed a bi-modal trend. The diurnal amplitude of ozone in summer, showed slight effect from the mountain and valley winds, but the amplitude around 2 ppbv is a lot smaller compared to other mountain data. The amplitude of other seasons did not show any significant variations. The effect of the mountain and valley winds were small at the top of Mt. Fuji because of the steep and windy summit. Day-to-day summertime variations varied wildly and often recorded low concentrations of ozone less than 20 ppbv. This low concentration of ozone sometimes continued for 3--5 days. This summer ozone minimum of the lower free troposphere was due to horizontal advection of the ozone poor air. In wintertime, the deviation of ozone concentration was very small. High peaks which lasted only a few hours were observed in spring. Analyzing the cross section chart on the 14th May 1993, the high peak of ozone on this day must have come from the stratosphere along the isentropic surface. ¿ American Geophysical Union 1994