PURPOSE: To perform enzymatic conversion of a scarcely water-soluble substrate in high efficiency by adsorbing and immobilizing an enzyme to a hydrophobic porous carrier and using the enzyme in the presence of an inorganic salt and a water-soluble polymeric substance.

CONSTITUTION: The enzymatic conversion of a scarcely water-soluble substrate (e.g., methone or methyl isobutyl carbinol) is carried out by adsorbing and immobilizing an enzyme (preferably lipase or 3α-hydroxysteroid dehydrogenase) to a hydrophobic porous carrier (e.g., activated carbon) and using the enzyme in the presence of an inorganic salt (especially preferably ammonium sulfate) and a water-soluble polymeric substance (preferably a polyethylene glycol having a molecular weight of 2,000-10,000). The amounts of the inorganic salt and the water-soluble polymeric substance are preferably 40-80wt.% and 2.5-20wt.%, respectively, based on the saturated state of the reaction liquid in the aqueous phase.