PROBLEM TO BE SOLVED : To provide a compound having aromatase inhibitory activity different from a steroidal compound or an azole compound.SOLUTION : An aromatase inhibitor comprises a compound of the following formula (1) (where, X, R1-R4 are as defined in the specifications), its isomer, pharmaceutically acceptable salt or a mixture of them as an active ingredient. The compound of the invention has aromatase inhibitory activity, and as an aromatase inhibitor, it is available as a medicine for the treatment and/or prevention of estrogen-dependent disease, particularly post-menopausal breast cancer.SELECTED DRAWING : None