To allow a safe operation, increase the service life of coolant, and protect a natural environment by efficiently regenerating the coolant in a short period of time without disposing off the coolant.

In these methods, the cleaning of the inside of a water-cooled engine system to which a thermostat is connected in series to the engine side of a hose for connecting an engine to a radiator and the purification treatment of the coolant are performed simultaneously. The upper hose 4 between the engine 1 and the radiator 2 is disconnected from the radiator side, the cleaning solvent feed hose 10 of a cleaning and purification system is connected to the hose 4, and a cleaning solvent suction hose 11 is connected to a connection port 7 on the radiator side. Then the cleaning and purification system is operated to forcibly open the thermostat 6 by a hydraulic pressure to circulate the coolant. As a pump, an air diaphragm pump can be used.