PURPOSE: The isomerization and oxidation of a purified glucose solution is effected using a fixed enzyme containing glucose isomerase and oxidase, thus producing continuously said solution of high fructose concentration.

CONSTITUTION: After a purified glucose liquid of more than 93wt% glucose content and 40W45wt% solid fraction is combined with 1×10-2W1×10-3mol/l magnesium ion and an alkali to ajust the pH to 7.5W8.2, the solution is continuously passed through a column filled with a fixed enzyme containing glucose isomerase at 50W 60°C to convent 45W50% of glucose into fructose. Then, the isomerized solution is aerated with oxygen or air and passed through a column in which a fluidized bed of a fixed enzyme containing glucose oxidase is formed at 30W40°C and a pH of 4W6 that is adjused by adding an alkali. Finally, gluconic acid formed is separated, the residue is purified and concentrated to give said solution of high fructose concentration.