The present invention relates to a process for producing a useful substance, which includes feeding a liquid mixture formed of two liquid phases into a fixed bed-type reaction column packed with an immobilized enzyme, and performing a reaction while allowing the liquid mixture to flow in the same direction in a co-current manner, wherein the fixed bed-type reaction column has partition plates inserted in the longitudinal direction of the fixed bed-type reaction column so as to form a plurality of tube-shaped structures, wherein each tube-shaped structure has a lateral cross-section which is circular or polygonal in shape with at least a part being unclosed and has a representative length of 100 mm or less, wherein the immobilized enzyme is packed into the tube-shaped structures, and wherein the liquid mixture is fed into the tube-shaped structures.Even when a reaction column having a large column diameter is used in the reaction performed by feeding a liquid mixture formed of two liquid phases into a fixed bed-type reaction column packed with an immobilized enzyme, the flow of the whole reaction liquid inside the column can be made uniform, and the enzymatic activity can be effectively appeared. As a result, reactivity and productivity can be enhanced. Particularly, in the hydrolysis of oils and fats, the enzymatic activity can be effectively appeared, so that fatty acids can be effectively produced. Furthermore, the operability at the time of removing the immobilized enzyme packed in the reaction column is good.