To provide a method for easily determining, in a highly sensitive way, glucose or ADP occurring or formed in a test solution of serum, plasma, urine, spinal fluid, etc.

This method comprises treating a test solution containing either glucose or ADP component with either of the remaining glucose or ADP component and with an enzyme protein catalyzing the reaction, and then, treating glucose-6-phosphate produced by the above reaction with glucose-6-phosphate dehydrogenase catalyzing a reaction by which glucono-δ-lactone-6-phosphate is produced, using an oxidized thio-NAD (P) or oxidized NAD (P) as a coenzyme and at least glucose-6-phosphate as a substrate and its reversible reaction, and with a reagent containing two kinds of coenzymes, i.e., an oxidized or reduced thio-NAD (P) and a reduced or oxidized NAD (P) to induce a cycling reaction, and thereby, determining the quantity of the coenzyme which may change due to the reaction.