In March 1966, factory plant wastewater was reported to be suspected as the source of pollution, and in September, the Ministry of Health and Welfare announced it had discovered methylmercury in moss at the outlet of the Showa Denko factory in Kanose village.
Oblast v prefekturi, univerza Niigata, državljanske organizacije in vsi lokalni ljudje so delali skupaj, da bi odkrili vzrok.