Physician and public health administrator. Born in Nagasaki as the son of a physician serving in the Hizen Omura Clan. In 1854 he entered the Tekijuku of Osaka, and later served as its head. In 1860, he went to Nagasaki where he learned Dutch medicine under the tutelage of Pompe. Coming to Tokyo in 1871, he became Monbu Shojo(officer of the Education Ministry) and joined Iwakura Mission to Europe, where he observed and investigated medical education in Western Europe. Returning to Japan in 1873, he was appointed the Chief of the Medical Affairs Bureau in Ministry of Education. In 1875, he became the first Director of the Sanitary Affairs Bureau, Home Ministry. Nagayo established the basis of Japan's public health administration, founded the Drug Control Laboratory, formulated a medical administration system, and introduced epidemic prevention and quarantine systems. He successively held important posts as a member of the Chamber of Elders and a member of the House of Peers. He wrote an autobiography titled "Shoko Shishi" (1902).