An air-conditioning system includes a heat source side refrigerant circuit in which a heat source side heat exchanger is provided, a load side heat medium circuit in which a load side heat exchanger is provided, an intermediate heat exchanger, and a heat medium sealing. The heat medium sealing mechanism includes a supply port through which the heat medium and gas flow, the gas being more soluble in the heat medium than air, a discharge port through which the gas pushed by the heat medium is discharged, and a flow straightener that is connected to the load side heat medium circuit in such a manner that, when the gas is supplied, the gas flows from the supply port to the discharge port, and when the heat medium is supplied, the heat medium flows from the supply port to the discharge port.