To reliably fill a defined amount of liquefied carbon dioxide when filling the liquefied carbon dioxide from a gas cylinder to a natural coolant heat pump water heater.

A filling device 1 is provided between the gas cylinder 2 and the water heater 3. A first supply line 4 extending from the gas cylinder 2 is connected to first and second cylinder chambers 6a, 6b of a double-acting cylinder 6. A second supply line 5 extending from the water heater 3 is connected to the first and second cylinder chambers 6a, 6b of the double-acting cylinder 6. Check valves 10a, 10b, 13a, 13b are provided at predetermined points of the first and second supply lines 4, 5. Accordingly, liquefied gas in the gas canister 2 always flows in the water heater 3 and the first and second cylinder chambers 6a, 6b, and pressurized gas in the double-acting cylinder 6 flows only in the water heater 3. When a gas pressure in the gas cylinder 2 and a gas pressure in the water heater 3 are equalized and the gas cylinder 2 and the water heater 3 cannot be filled any more, they are filled by operating the double-acting cylinder 6.