To clarify the effects of ozone (O3) on the resistance of Japonica rice (Oryza sativa L.) to root lodging, the Japanese major rice cultivar Koshihikari was exposed to four levels of O3 in open-top chambers (mean O3 concentration: 17.3, 32.4, 51.1, and 66.3 nl l-1). We evaluated the resistance to root lodging by measuring pushing resistance at 15 days after heading (DAH) and the dry masses of plant organs such as roots at harvest (35 DAH). Exposure to O3 significantly reduced pushing resistance per panicle, suggesting that the O3 reduced resistance to root lodging. Because the degree of O3-induced reduction in pushing resistance per panicle correlated with that of O3-induced reduction in root dry mass per panicle, the O3-induced reduction in the resistance to root lodging could be caused by O3-induced reduction in root development. There was a significant negative correlation between relative value of pushing resistance per panicle and O3 concentration or dose, indicating that resistance to root lodging decreases with increasing O3 concentration. In East Asia, because the O3 concentration is projected to increase in the near future, lodging issues in the rice cultivation could become serious.