PURPOSE: To obtain an optically active 1,3-dioxane derivative in high optical purity by treating a specific compound with a fungus and stereoselectively reducing.

CONSTITUTION: 0.01-20wt.% compound shown by formula I (R is 2-20C alkyl or alkylene-substituted methylene; R1 is 1-4C alkyl, alkenyl or haloalkyl) is blended with a carbon source such as sucrose, a fungus such as baker's yeast and city water, etc., to give a reaction solution (A). Then the component A is asymmetrically reacted at 10-60°C for 1-100 hours to give a reaction product (B). Then the component B is mixed with salt, sellaite, made into paste, extracted with chloroform, the prepared extract is purified by column chromatography, etc., to produce an optically active 1,3-dioxane derivative (C) shown by formula II (* is asymmetric carbon). Optionally the component C is heated in an organic solvent such as toluene to yield an optically active compound shown by formula III.