The present invention relates to a process for producing a useful substance, the process includes feeding a liquid mixture formed of two liquid phases into a fixed bed-type reaction column packed with an immobilized enzyme, and allowing the liquid mixture to flow in the same direction in a co-current manner to perform a reaction. The packing thickness of the immobilized enzyme per stage of the fixed bed-type reaction column having an equivalent circular diameter of 35 mmφ or larger is 10 to 200 mm. The flow of the reaction liquid inside the column can be made uniform, and as a result, reactivity and productivity is enhanced.