It has been found that a compound of the general formula (I) having an isoxazole skeleton has excellent inhibitory activity against mutant IDH1 protein and inhibits the production of 2-HG by this protein, while the compound is also capable of effectively inhibiting the growth of various tumors expressing the protein. In the formula, R 1 , R 2 , R 3 , Y, and Z are as defined in claim 1.