A process for preparing an optically active sulfoxide derivative (I) having CCR5 antagonism without causing side reactions such as racemization and Pummerer rearrangement, which comprises reacting a compound (II) with a compound (III) as shown by the following scheme: wherein R1 represents hydrogen, an aliphatic hydrocarbon group or an aromatic group; R2 represents halogeno, alkyl, hydroxyl, amino, an aromatic group, etc.; R3 represents a 5- or 6-membered ring; R4 represents hydrogen, alkyl, alkoxy or halogeno; R5 represents hydrogen, a hydrocarbon group, a heterocyclic group, acyl, etc.; ring A represents an optionally substituted benzene ring; X represents a bond or divalent group comprising a linear part constituted of 1 to 4 atoms; m represents an integer of 1 to 5; n represents an integer of 0 to 3; p represents an integer of 0 to 2; and *1 represents an asymmetric center.