A 1,2-benzoisothiazole derivative or a salt thereof is not harmful to a crop and is remarkably effective against agricultural or horticultural plant diseases such as Pyricularia oryzae, Septoria nodorum, Pseudoperonospora cubensis and Colletotrichum orbiculare, and an agricultural or horticultural plant disease-controlling agent containing the derivative or its salt as an active ingredient. The 1,2-benzoisothiazole derivative is represented by the general formula [I]: ##STR00001## (wherein R.sup.1 is a hydrogen atom or a halogen atom, and R.sup.2 is a methyl group or a halogen atom), or a salt thereof.