PURPOSE: To reduce a compound without by-products at ordinary temperature and normal pressure by reacting a carboxylic acid, a CO compound, an SO compound, etc., with a reductase-containing resting microorganism in a biphase medium composed of aqueous medium and organic medium under an atmosphere such as H2 or CO in the presence of an electron transmitter.

CONSTITUTION: At least one compound of carboxylic acids (e.g. propionic acid) and the salts, carbonyl compounds (e.g. benzaldehyde) and sulfoxide compounds (e.g. methylphenylsulfoxide), etc., is reduced using a resting cell of a microorganism having a reductase (Clostridium thermoacetycum ATCC35608) in a liquid biphase system composed of an aqueous medium and substantially water- insoluble or slightly water-soluble organic solvent (e.g. isopropyl ether) under an atmosphere of H2 and/or CO in the presence of an electron transmitter (e.g. methyl viologen) having -100to -500mV standard oxidoreduction potential. Thereby, the compound is efficiently reduced by a simple operation almost without producing by-products at an ordinary temperature and normal pressure.