This invention provides a process for producing optically active 2-[6-(hydroxymethyl)-1,3-dioxan-4-yl]acetic acid derivatives, which are of value as intermediates of drugs, from inexpensive starting materials without using any special equipment such as that required for super-low temperature reactions.A process for producing optically active 2-[6-(hydroxymethyl)-1,3-dioxan-4-yl]acetic acid derivatives which comprises reacting an acetic acid derivative at a temperature of not less than −30° C. with an enolate prepared by permitting either a base or a metal having a valency of 0 to act on the derivative to produce a hydroxyoxohexanoic acid derivative, reducing this compound with the aid of a strain of microorganism to provide a halomethyldioxanylacetic acid derivative, treating this compound with an acetalizing agent in the presence of an acid catalyst to provide a halomethyl-dioxanylacetic acid derivative, reacted with an acyloxylating agent to provide a acyloxymethyldioxanylacetic acid derivative, and subjecting this compound to solvolysis in the presence of a base.