The present invention provides a compound having a steroid C.sub.17,20-lyase-inhibitory activity and useful for the therapy and prophylaxis of tumor such as prostatism, breast cancer and the like, and a method for efficiently separating an optically active compound of this compound from a mixture of optical isomers thereof, a compound of the formula: ##STR00001## wherein each symbol is as defined in the specification, a salt thereof or a prodrug thereof, and a method for obtaining an optically active compound by optically resolving a mixture of optical isomers by the use of a resolving agent such as tartranilic acid and the like.