A core forming device is equipped with a kneading tank in which raw materials of a core are kneaded, a raw material supply unit that supplies the raw materials to the kneading tank, a mold that accommodates a kneaded material including the raw materials kneaded in the kneading tank and that forms the core, a piston that injects the kneaded material into the mold, a position sensor that detects a position of the piston, and a control unit that controls a supply amount of the raw materials supplied to the kneading tank from the raw material supply unit. The control unit determines the supply amount of the raw materials based on a difference between the position of the piston upon completion of injection and a reference position of the piston.