We present the guideline for use of [At] sodium astatide (NaAt) for targeted alpha therapy in clinical trials on the basis of radiation safety issues in Japan. This guideline was prepared by a study supported by the Ministry of Health, Labour, and Welfare, and approved by the Japanese Society of Nuclear Medicine on 8th Feb, 2021. The study showed that patients receiving [At]NaAt do not need to be admitted to a radiotherapy room and outpatient treatment is possible. The radiation exposure from the patient is within the safety standards of the ICRP and IAEA recommendations for the general public and caregivers. Precautions for patients and their families, safety management associated with the use of [At]NaAt, education and training, and disposal of medical radioactive contaminants are also included in this guideline. Treatment using [At]NaAt in Japan should be carried out according to this guideline. Although this guideline is applied in Japan, the issues for radiation protection and evaluation methodology shown here are considered internationally useful as well.