An object of the present invention is to provide a control material which can be preferably employed in detection of 1,5-anhydro-D-glucitol and glycoalbumin, which are employed as excellent indices for diabetes. The present inventor has found that mannitol which is added to serum or plasma used as control material stabilizes 1,5-anhydro-D-glucitol and glycoalbumin present in the serum or plasma for a long period of time, and that the object can be attained through provision of (1) an agent for stabilizing control material, the agent being composed of mannitol and (2) control material containing mannitol and 1,5-anhydro-D-glucitol.