Kōshi is a city in Kumamoto Prefecture, Japan. As of 31 July 2024, the city had an estimated population of 65,054 in 26670 households, and a p…Kōshi is a city in Kumamoto Prefecture, Japan. As of 31 July 2024, the city had an estimated population of 65,054 in 26670 households, and a population density of 860 persons per km². The total area of the city is 53.19 km²..