An SO2 continuous monitor consisting of a commercially available pulsed UV fluorescence instrument with zero and span gas calibration was installed at the summit of Mt. Fuji (3776 m asl) in September 2002. The system produces data with a time resolution comparable with other trace gases. The instrumental feasibility was tested onsite, and the SO2 concentration level at the summit was thereafter routinely observed. The present detection capability of the system, expressed in terms of the critical level (Lc, definition by International Union of Pure and Applied Chemistry and International Organization for Standardization), was estimated to be about 0.05 ppbv. Thus it was difficult to observe the temporal change of very low background SO2. However, the system is satisfactory for observing episodic transport of SO2, particularly during winter. No high SO2 episodes were observed during summer, in contrast to winter. One extraordinary episode was observed in late October 2003, the only one attributable to the Miyake-jima SO2 volcanic plume. High SO2 episodes were more evident (with longer duration and higher concentration level) in February 2003 among the winter months observed. Typical February conditions were determined using backward trajectory, a surface weather map, and other indicators. A comparison of the temporal changes in SO2, CO, and 222Rn concentrations in the winter months suggests that these gases in the free troposphere over Japan may have been transported together in most cases from the same source regions somewhere in the Asian continent. The correlation between SO2 and 222Rn in such episodes may also suggest a short timescale for transport from the source to Mt. Fuji of within a few days. The chemical time series data of SO2 at Mt. Fuji is important for understanding the free tropospheric chemical nature, such as the Asian outflow over the North Pacific.