To provide a cooling system in which air storage does not occur in a refrigerant passage on the thermostat side, and to provide a refrigerant filling method and a discharging method in this cooling system.

The cooling system is arranged in a refrigerant passage between a cooling object and a radiator, and includes a thermostat for controlling the circulation of a refrigerant by opening-closing a valve element in response to the temperature of the refrigerant. In the cooling system, the thermostat 10 has a jiggle valve 22, and becomes the air bleeding side via the jiggle valve 22 when filling the refrigerant. An air storage part 12 is formed in a position higher than the jiggle valve 22 on a refrigerant passage wall on the jiggle valve 22 side, and the air storage art 12 is provided with an air bleeding hole 13 for exhausting the stored air when filling the refrigerant.

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