To efficiently obtain an (S) oxetin enantiomer of a high purity by carrying out two specific steps when its racemic compound is prepared from a propiophenone via a racemic alcohol.

(A) The propiophenone represented by formula I (X is a halogen, OH or the like) is selectively converted to (B) racemic alcohol of formula II under the non-chiral conditions and then, (C) the racemic compound of formula III (R1 is H, a 1-5C alkyl; R2 is a 1-5C alkyl; Y is 4-trifluoromethylphenyl or the like) is obtained. At this time, the following two steps are performed to the objective enantiomer: in the first step, the component B is optically resolved by the simulated moving bed continuous chromatography using a chiral adsorbent into the first enantiomer with an enantiomer purity of ≥51% and the second enantiomer; in the second step, the first enantiomer alcohol instead to the component B is converted to the component C having (S) conformation with an enanitomer purity of ≥95%.