Rice is one of the most important crops in the world. Rice blast disease caused by Magnaporthe oryzae is one of three major destructive diseases of rice. Chemical fungicides are required for the control of rice blast. However, overcoming fungicide resistance of M. oryzae is critical for successful chemical control. In this study, the inhibitory activity of hot water extracts of buckwheat straw against M. oryzae was evaluated for new fungicide development. Germination of M. oryzae conidia was significantly inhibited after incubation for 24 h in an extract of buckwheat straw. Significantly fewer blast lesions were formed in M. oryzae inoculated seedlings treated with the extract compared to seedlings treated with distilled water. These results indicate that inhibitory substances effective against M. oryzae may be present in hot water extracts of buckwheat straw.