In Japan, all 47 prefectures conduct routine air quality monitoring at 1,145 stations throughout the country to assess the effects on the environment. The current study aims to provide a better understanding of possible estimations of crop yield loss caused by exposure to monitored concentrations of surface ozone in Japan during the key ozone-season months from April to December 2005. The effect of surface ozone on the irrigated rice crop in Japan is evaluated based on the AOT40 index (accumulated exposure over threshold of 40 ppb) during 3-month periods of the growing season. Assuming no changes in land use and field management practices during the period from 2000 to 2005, we find that the estimated average relative yields of the irrigated rice crop is 91 % caused by exposures to monitored surface ozone concentrations. These losses in the relative yield of the irrigated rice crop in Japan in 2005 could cost around 280.4 million (int. \$).