PURPOSE: To obtain the subject compound useful as a production raw material for poly-3-hydroxybutyrate, etc., useful as a biodegradable polymer on an industrial scale at a low cost by using a cultured product of a microbial strain belonging to the genus Bacillus or treated cell of the microbial strain.

CONSTITUTION: 3-Hydroxybutyric acid and/or its trimer useful as a production raw material for biodegradable polymer, etc., are produced by the following microbial reaction and/or enzymatic reaction. A microbial strain belonging to the genus Bacillus and capable of producing 3-hydroxybutyric acid and/or its trimer [e.g. Bacillus megatherium B-124 (FERM P-12803)] or its treated cell is added to a liquid medium consisting of sucrose, NH4Cl, NaCl, K2HPO4, KH2PO4, MgSO4.7H2O, corn steep liquor and distilled water and cultured at 30°C under shaking, the cultured product is extracted with ethyl acetate and the solvent is distilled off from the extracted liquid.