PROBLEM TO BE SOLVED: To obtain the subject compound having a high affinity to a dopamine D4 receptor, excellent in wide use in general clinic and useful as a radioactive diagnostic agent by using a specific non-radioactive bromine-substituted benzamide derivative as a raw material.
SOLUTION: This radioactive iodine-labeled diagnostic agent to be bonded to a dopamine D4 receptor is obtained by introducing 123I by using a non- radioactive bromine-substituted benzamide derivative expressed by formula I as a raw material in a halogen exchanging reaction to obtain an I123-labeled- bnzamide derivative, e.g. (S)-N-(1-benzyl-3-pyrrolidinyl)-4-[(cyclopropylcarbonyl) amino]-5-iodo-2-methoxybenzamine. Further, e.g. it is preferable that the above reaction is a bromine-radioactive iodine exchanging reaction and uses ammonium sulfate, etc., as a catalyst.