A lactose-containing solution such as a lactose aqueous solution, wherein milk or the like is placed in contact with a β-galactosidase-containing solution, with a dialysis membrane intervening therebetween to decompose lactose in the solution. Thus, a β-galactosidase-free solution of the lactose decomposition product is obtained. This enzymatic reaction is conducted at temperatures not higher than the optimal temperature of used enzyme, preferably 0-10°C, with the amount of enzyme being 30 to 250 units, preferably 62.5-250 units per 1 g of lactose in the lactose-containing solution. The area of the dialysis membrane intervening between the enzyme-containing solution and the substrate-containing solution is preferably selected so that it actually becomes largest with respect to the volume of the enzyme-containing solution used.