Method for manufacturing cast product

A method for manufacturing a cast product in which a core can be easily discharged from a casting roughly-shaped material is provided. A method for manufacturing a cast product according to the present disclosure includes: a step of hermetically sealing a casting roughly-shaped material formed by casting using a core in a container containing water; a step of heating the water contained in the container while keeping the casting roughly-shaped material hermetically sealed and thereby destroying the core while keeping a pressure inside the container at a pressure higher than an atmospheric pressure, and thereby forming core sand; and a step of discharging the core sand from the casting roughly-shaped material.