An air conditioning system includes a refrigerant circuit (20) in which a compressor (21), an indoor radiant panel (23), a first expansion valve (24), a room air heat exchanger (25), a second expansion valve (26) and an outdoor air heat exchanger (27) are connected in this order and which operates in a refrigeration cycle by reversibly circulating refrigerant therethrough. During a defrosting operation, the first expansion valve (24) is controlled to reduce the refrigerant pressure so that in a cooling cycle the refrigerant releases heat in the outdoor air heat exchanger (27) and the room air heat exchanger (25) and evaporates in the indoor radiant panel (23). Thus, the air conditioning system concurrently provides the defrosting of the outdoor air heat exchanger (27) and the room heating of the room air heat exchanger (25).