It is intended to provide a radioactive diagnostic imaging agent comprising a radioactive halogen-labeled compound as an active ingredient, in which the active ingredient is prevented from radiolysis and its stability is improved. This is achieved by adding a biologically-acceptable sugar or sugar alcohol to the radioactive diagnostic imaging agent in an amount effective to prevent radiolysis. The amount of the sugar or sugar alcohol to be added is preferably 10 (mmol/L)/GBq/mL or more, and more preferably 50 (mmol/L)/GBq/mL or more. The sugar is preferably selected from the group consisting of erythrose, threose, ribose, arabinose, xylose, lyxose, allose, altrose, glucose, mannose, gulose, idose, galactose, talose, erythrulose, ribulose, xylulose, psicose, fructose, sorbose, and tagatose. The sugar alcohol is preferably selected from the group consisting of erythritol, xylitol, sorbitol, and mannitol.