[en] The radiological feature of the radioactive material leak incident at the J-PARC Hadron experimental facility has been investigated. From the systematic analysis focusing on the species and behavior of radionuclides originated in the gold target that had been used for nuclear and particle physics experiments since December 14, 2012, we have figured out the major process that resulted in the release of radioactive materials into the user working areas in the Hadron experimental hall as follows; 1) a variety of radionuclides had been produced in the gold target via nuclear spallation reactions, 2) radionuclides with boiling points lower than the melting points of gold were mainly discharged from the gold target because of the instantaneous overheat which raised the temperature of the gold target above the melting point of gold by injection of the anomalously intense-pulsed beam, 3) the radionuclides diffused in the primary beam line room by the air circulation system and leaked into the user working area through the inadequate hermetic sealing for cables and bulk shields between the primary beam line room and the user working areas. (author)