Masao Mimatsu (三松正夫; 9 July 1888 – 8 December 1977) was a Japanese postmaster who recorded the growth of the Shōwa-shinzan mountain in 1944–1945. On 31 December 1943, Shōwa-shinzan began forming from rapid uplifting of a wheat field as a result of a sudden earthquake. This seismic event would eventually change into volcanic activity and result in the eruption of Mount Usu in 1944. Due to the Japanese war effort, basic scientific materials were unavailable. However, Mimatsu, the postmaster of Sobetsu-cho, recorded measurements and drew diagrams of the rising mountain on paper.