PURPOSE: To determine the subject compound useful as a marker for diagnosing diabetes by treating 1,5-anhydroglucitol contained in a specimen with a phosphorylation enzyme in the presence of a phosphoryl group donor and subsequently measuring the amount of the consumed phosphoryl group donor.

CONSTITUTION: This method for determining 1,5-anhydroglucitol comprises treating 1,5-anhydroglucitol contained in a specimen prepared by diluting a blood, serum, plasma, urine, etc., collected from a living body with a 50mM phosphoric acid buffer solution (pH: 7.0) with a 1,5-anhydroglucitol-phosphory1 ation enzyme such as hexokinase or glucokinase in the presence of a phosphoric acid donor (e.g. adenosine triphosphate) to produce 1,5-anhydroglucitol phosphate and adenosine diphosphate (ADP), reacting the produced ADP with phosphoenol pyruvic acid in the presence of pyruvic acid kinase, oxidizing the produced pyruvic acid in the presence of an oxidization enzyme, reacting the produced H2O2 with a color source substance in the presence of a peroxidase, and subsequently measuring the produced pigment.