In order to clarify the long-term variation of photochemical oxidants (O_x) over Japan, we analyzed the O_x concentrations measured at monitoring stations over Japan from 1985 to 1999 fiscal year. We found that the O_x concentration observed at 82% of all stations tended to increase during 15 years and its increase rate averaged over Japan was 0.33 ppb per year (1.1% per year). Especially, the increase of O_x concentration from 1991 to 1996, i.e. about 5ppb, was remarkably large. Furthermore, statistically significant O_x increases have been deduced all over Japan and the increases tend to be slightly large in spring and summer seasons except June. However, it must be noted that the application of an O_x analyzer with auto-washing equipment may have affected the O_x long-term variations.