A cooling system for cooling an internal combustion engine includes a first portion, a second portion and a bypass valve. The first portion forms a first fluid chamber through which a cooling medium passes as it flows from the engine to a heater that utilizes heat generated by the internal combustion engine. The second portion forms a second fluid chamber through which the cooling medium passes as it flows from the heater to the engine. The second fluid chamber is arranged adjacent to the first fluid chamber. The bypass valve is arranged inside the first fluid chamber and the second fluid chamber to form a bypass flow passage connecting the first fluid chamber and the second fluid chamber. The bypass valve is configured and arranged to automatically close the bypass flow passage when a temperature of the cooling medium is equal to or higher than a first prescribed temperature.