In September 1905, Hong Sa-ik entered the Korean empire military school but switched to a Japanese military school in Tokyo after Japan took over the country. He joined the Japanese military. From 1910 until 1936, he served in the army and he was promoted to general. In 1940, he was appointed to work in the city of Nanjing in China. In 1944 he was sent to the Philippines, where he was put in charge of prisons for captured Allied soldiers.