Some derivatives of 3-oxo-1,2-benzisothiazole-1,1-dioxide were synthesized, and their antifungal activities against Pyricularia oryzae was determined by the agar medium dilution method. I_(50) values of the candidate derivatives were shown to be as follows; 3-chloro(37.8ppm), 2-chloro(318.7ppm), MCS(20.1ppm), 2-allyl(946.2ppm), 3-(p-nitrophenyloxy) (35.4ppm), 3-(o-nitrophenyloxy) (11.8ppm), 3-(p-aminophenyloxy) (1643.2ppm), 2-chloromethyl(192.7ppm) and 2-hydroxymethyl derivative(248.4ppm).