An air-conditioning system comprising: a compressor (101A, 101B) configured to compress a refrigerant; an indoor heat exchanger configured to allow the refrigerant to exchange heat with indoor air; an outdoor heat exchanger configured to allow the refrigerant to exchange heat with outdoor air; a four-way valve configured to guide the refrigerant discharged from the compressor (101A, 101B) to any one of the indoor heat exchanger and the outdoor heat exchanger; a first connection pipe configured to connect the outdoor heat exchanger and the indoor heat exchanger; a second connection pipe configured to connect the indoor heat exchanger and the four-way valve; an outdoor expansion valve disposed on the first connection pipe and configured to allow the refrigerant to be decompressed and expanded before being transferred to the outdoor heat exchanger during heating; and a reservoir configured to store the refrigerant, and characterised in that: the reservoir is disposed on the first connection pipe between the outdoor heat exchanger and the outdoor expansion valve, and the first connection pipe comprises: a first pipe portion (R5-1) having one end connected to the outdoor heat exchanger and the other end connected to a lower end of the reservoir, and a second pipe portion (R5-2) having one end connected to the outdoor expansion valve and the other end connected to a lower portion of the reservoir at a higher level than the other end of the first pipe portion (R5-1), and a gaseous refrigerant guide pipe (R5-3) having one end connected to an upper end of the reservoir and the other end connected to the second pipe portion (R5-2) and configured to guide a gaseous refrigerant. a compressor (101A, 101B) configured to compress a refrigerant; an indoor heat exchanger configured to allow the refrigerant to exchange heat with indoor air; an outdoor heat exchanger configured to allow the refrigerant to exchange heat with outdoor air; a four-way valve configured to guide the refrigerant discharged from the compressor (101A, 101B) to any one of the indoor heat exchanger and the outdoor heat exchanger; a first connection pipe configured to connect the outdoor heat exchanger and the indoor heat exchanger; a second connection pipe configured to connect the indoor heat exchanger and the four-way valve; an outdoor expansion valve disposed on the first connection pipe and configured to allow the refrigerant to be decompressed and expanded before being transferred to the outdoor heat exchanger during heating; and a reservoir configured to store the refrigerant, and characterised in that: the reservoir is disposed on the first connection pipe between the outdoor heat exchanger and the outdoor expansion valve, and the first connection pipe comprises: a first pipe portion (R5-1) having one end connected to the outdoor heat exchanger and the other end connected to a lower end of the reservoir, and a second pipe portion (R5-2) having one end connected to the outdoor expansion valve and the other end connected to a lower portion of the reservoir at a higher level than the other end of the first pipe portion (R5-1), and a gaseous refrigerant guide pipe (R5-3) having one end connected to an upper end of the reservoir and the other end connected to the second pipe portion (R5-2) and configured to guide a gaseous refrigerant. the reservoir is disposed on the first connection pipe between the outdoor heat exchanger and the outdoor expansion valve, and the first connection pipe comprises: a first pipe portion (R5-1) having one end connected to the outdoor heat exchanger and the other end connected to a lower end of the reservoir, and a second pipe portion (R5-2) having one end connected to the outdoor expansion valve and the other end connected to a lower portion of the reservoir at a higher level than the other end of the first pipe portion (R5-1), and a gaseous refrigerant guide pipe (R5-3) having one end connected to an upper end of the reservoir and the other end connected to the second pipe portion (R5-2) and configured to guide a gaseous refrigerant. a first pipe portion (R5-1) having one end connected to the outdoor heat exchanger and the other end connected to a lower end of the reservoir, and a second pipe portion (R5-2) having one end connected to the outdoor expansion valve and the other end connected to a lower portion of the reservoir at a higher level than the other end of the first pipe portion (R5-1), and a gaseous refrigerant guide pipe (R5-3) having one end connected to an upper end of the reservoir and the other end connected to the second pipe portion (R5-2) and configured to guide a gaseous refrigerant.