An air conditioning system comprising: a heat source unit capable of adjusting a cooling capacity or a heating capacity for generating cold or hot water; an indoor unit configured to exchange heat between sucked air and the cold or hot water and blow out the air; a water circuit configured by connecting a supply pipe and a return pipe so that the cold or hot water circulates to the heat source unit and the indoor unit; a flow rate adjusting valve provided in the water circuit and capable of adjusting a flow rate of the cold or hot water; a supply air temperature control unit configured to adjust a flow rate of the flow rate adjusting valve; a pump provided in the water circuit, of which rotation speed is adjustable; a pump controller configured to control the rotation speed of the pump; a return water temperature sensor configured to detect a temperature of the cold or hot water flowing through the return pipe; a supply water temperature sensor configured to detect a temperature of the cold or hot water flowing through the supply pipe; a supply water temperature control unit configured to adjust the cooling capacity or the heating capacity of the heat source unit so that the supply water temperature detected by the supply water temperature sensor becomes a target supply water temperature; and a target supply water temperature updating unit configured to change the target supply water temperature based on a temperature difference between the return water temperature detected by the return water temperature sensor and the supply water temperature, characterized in that, when the heat source unit cools the cold or hot water, the target supply water temperature updating unit is configured to decrease the target supply water temperature when the temperature difference between the return water temperature and the supply water temperature is smaller than a preset first threshold value. a heat source unit capable of adjusting a cooling capacity or a heating capacity for generating cold or hot water; an indoor unit configured to exchange heat between sucked air and the cold or hot water and blow out the air; a water circuit configured by connecting a supply pipe and a return pipe so that the cold or hot water circulates to the heat source unit and the indoor unit; a flow rate adjusting valve provided in the water circuit and capable of adjusting a flow rate of the cold or hot water; a supply air temperature control unit configured to adjust a flow rate of the flow rate adjusting valve; a pump provided in the water circuit, of which rotation speed is adjustable; a pump controller configured to control the rotation speed of the pump; a return water temperature sensor configured to detect a temperature of the cold or hot water flowing through the return pipe; a supply water temperature sensor configured to detect a temperature of the cold or hot water flowing through the supply pipe; a supply water temperature control unit configured to adjust the cooling capacity or the heating capacity of the heat source unit so that the supply water temperature detected by the supply water temperature sensor becomes a target supply water temperature; and a target supply water temperature updating unit configured to change the target supply water temperature based on a temperature difference between the return water temperature detected by the return water temperature sensor and the supply water temperature, characterized in that, when the heat source unit cools the cold or hot water, the target supply water temperature updating unit is configured to decrease the target supply water temperature when the temperature difference between the return water temperature and the supply water temperature is smaller than a preset first threshold value.