PURPOSE:To readily and rapidly determine the optical purity of the subject compound useful as a raw material of an intermediate of medicines and optically active functional materials by carrying out direct optical resolution of an enantiomer of the subject compound using a separating agent composed of a polysaccharide or a derivative thereof as the active component. CONSTITUTION:Using a separating agent composed of a polysaccharide (preferably homoglucane having high regularity and constant type of bond, especially cellulose, amylose, beta-1,4-chitosan, etc., capable of ready preparation of high- purity polysaccharide) or a derivative thereof in which H on the hydroxyl groups is partially or wholly, preferably in an amount of >=85%, substituted with another atomic group as the active component, an enantiomer of a compound of the formula (R is alkyl or nonsubstituted or substituted aromatic group; X is acyloxy, aryloxy, alkylthio, arylthio or non-substituted, mono- or disubstituted amino) is directly optically separated by the liquid chromatography, etc.