To provide a cooling device and a cooling method capable of easily treating fly ash.

This cooling device used for a conventional cooling device cooling an exhaust gas generated in burning a flammable material, comprises a cavity casing as a casing surrounding a cooling space through which the exhaust gas is passed, an exhaust gas temperature measuring device for measuring an exhaust gas temperature as a temperature of the exhaust gas at an exhaust gas temperature measuring position in the flow of the exhaust gas in the cooling space, and a water spraying device for spraying the water to a water spraying position at on upstream side with respect to the exhaust gas temperature measuring position in the flow of the exhaust gas in the cooling space, and the quantity of sprayed water of the water spraying device is adjusted so that the exhaust gas temperature is not over a specific temperature.