To provide a method for producing cystacyclin useful as peripheral nerve-treating agent in good yield by using a cyclopentanone derivative in which only an ester group at 2-position in two ester groups is changed to a thioether group.

The objective compound is produced by optionally protecting functional groups of (2S,3R)-3-protected hydroxymethyl-2- thiomethylcyclopentanone derivative of formula I [R1 is a protecting group of hydroxy group; R2 is a 1-9C hydrocarbon residue (substituted by a halogen, nitro, cyano, hydroxyl, amino, carboxyl, etc.)] or a salt thereof, thereafter selectivity deprotecting the protected hydroxymethyl group at 3-position, oxidizing the thiomethyl group at 2-position to provide carboxyl group and to obtain a compound of formula II, further reacting the compound of formula II with N-protected-L-cysteine and optionally deprotecting the protected groups to provide the objective compound.