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.