A method for producing a radioactive diagnostic imaging agent which contains a radioactive halogen-labeled amino acid compound as an effective ingredient and has a composition that can inhibit radiolysis of the effective ingredient, thereby further improving stability, is provided. Disclosed is a method for producing a radioactive diagnostic imaging agent, comprising a solution preparation step of preparing a solution containing a radioactive halogen-labeled amino acid compound, and a dilution step of diluting the solution containing the radioactive fluorine-labeled organic compound in order to adjust radioactive Concentration thereof, which further comprises an acid addition step of adding an acid to the solution containing the radioactive fluorine-labeled organic compound, after the solution preparation step and before the dilution step, wherein the acid is added in the acid addition step in an amount sufficient to adjust the pH of the solution resulting from the dilution step to 2.0-5.9, for example, in an amount corresponding to 0.40-2.8 mmol per 1L of the solution resulting from the dilution step.
