A prenatally-exposed survivor of Hiroshima, Hamasumi was involved in the establishment of an association of atomic bomb survivors in Inagi City, Tokyo, in 2003 and has served as its president since 2012. As Deputy Secretary General of Nihon Hidankyo, he submitted the International Hibakusha Signatures to the United Nations Headquarters in New York in October 2018. In December 2024, Hamasumi attended the Nobel Peace Prize ceremony in Oslo.