Using a 1,5-anhydroglucitol dehydrogenase capable of acting on 1,5-anhydroglucitol and directly catalyzing a reducing chromophoric agent in the absence of an electron carrier, the 1,5-anhydroglucitol dehydrogenase is allowed to act on 1,5-anhydroglucitol in the presence of the reducing chromophoric agent preferably after the glucose in the specimen has been changed, or while being changed, into such a structure that it does not react with the 1,5-anhydroglucitol dehydrogenase in the specimen, by the aid of a glucose eliminator. Then, the amount of the resultant reduced colored substance is measured. As the 1,5-anhydroglucitol dehydrogenase, an enzyme produced by a microorganism having the ability to produce the 1,5-anhydroglucitol dehydrogenase is preferably used.