(+)-Lactose → (2R,3S,4R,5R)-2,3,4,5,6-pentahydroxyhexanal

In 30 ml of lactose solution having a concentration of 13.3 (W/V) % prepared by dissolving lactose in a 0.05 M acetate buffer solution of pH 4.5 was immersed 0.2-0.25 ml of an immobilized lactase. The reaction was allowed to proceed at 30° C. under reciprocating shaking (more than 100 rpm with an amplitude larger than 3.5 cm) for 15 minutes. Then the amount of glucose produced was determined with a glucose oxidase-peroxidase-dye system. The amount of enzyme which produces 1μ mol of glucose per minute is defined as one unit (1 ILU). The dry weight of an immobilized lactase is determined as mentioned below. That is, after the termination of the reaction, the immobilized lactase was filtered, dried at 50° C. under reduced pressure for 8 hours or more, placed in a desiccator at room temperature (18°-25° C.) for 1.5 hours or more, the weight thereof was then measured and this value is regarded as the dry weight of the immobilized lactase. The activity of an immobilized lactase is represented by the unit per gram of dry weight (1 ILU/g-IML).