Provided is a composition containing a buffer to be used at the time of labeling of a chelated targeting agent with 90Y, 153Sm, 165Dy, 165Er, 166Ho, or 177Lu. At least one kind of buffer selected from the group consisting of benzoic acid, maleic acid, fumaric acid, succinic acid, and salts thereof is incorporated in a composition containing a chelated targeting agent.