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.以放射性鹵素標示氨基酸化合物為有效成分之放射性影像診斷劑中,提供能抑制該有效成分的放射線衰變、可以具有更高安定性的組成物的製造方法。提供放射性影像診斷劑的製造方法,包含:調製含放射性鹵素標示氨基酸化合物的溶液的放射性氟元素標示有機化合物溶液調製作業、及稀釋前述含放射性氟元素標示有機化合物的溶液來調整放射能濃度的稀釋作業。該製造方法中,前述含放射性氟元素標示有機化合物溶液調製作業後、前述稀釋作業前,含有於前述含放射性氟元素標示有機化合物溶液中添加酸的酸添加作業,前述酸添加作業所添加的酸之量,是使稀釋作業後的溶液酸鹼值成為2.0至5.9的充分的量,例如,相對於稀釋作業後的溶液1公升,使用相當0.40~2.8毫莫耳,以此為特徵的方法。
