The invention provides a method for producing a radiation diagnostic imaging agent comprising an amino acid compound labeled with a radioactive halogen as an active ingredient, which hardly causes the radiation-induced degradation of the active ingredient and has such a composition that the stability can be improved. The invention discloses a method for producing a radiation diagnostic imaging agent, which comprises the following steps: a radioactive fluorine-labeled organic compound solution preparation step for preparing a solution containing an amino acid compound labeled with a radioactive halogen; and a dilution step for diluting the solution containing the organic compound labeled with the radioactive fluorine to adjust the radioactive concentration in the solution, wherein the method further comprises an acid addition step for adding an acid to the solution containing the organic compound labeled with the radioactive fluorine after the radioactive fluorine-labeled organic compound solution preparation step and prior to the dilution step, and the amount of the acid to be added in the acid addition step is a sufficient amount for adjusting the pH value of the solution after the dilution step to 2.0 to 5.9, for example, 0.40 to 2.8 mmol per 1 L of the solution after the dilution step.