An air conditioning system comprising: a cooling tower configured to cool cooling water; a thermal source device configured to generate cold water by the cooling water cooled by the cooling tower; a cooling water circulation passage configured to allow circulation of the cooling water between the cooling tower and the thermal source device; a first heat exchanger configured to exchange heat between air and the cold water generated by the thermal source device; a cold water circulation passage configured to allow circulation of the cold water between the thermal source device and the first heat exchanger; a second heat exchanger configured to exchange heat between a portion of the cooling water cooled by the cooling tower and the air heat-exchanged with the cold water by the first heat exchanger; the air conditioning system characterized by further comprising a cooling water branch passage (A'), branching from the cooling water circulation passage, configured to (i) introduce to the second heat exchanger the portion of the cooling water introduced to the thermal source device from the cooling tower, and (ii) introduce to the cooling water circulation passage the portion of the cooling water heat-exchanged with the air by the second heat exchanger; and flow path switching means configured to switch a flow path of the cooling water in the cooling water circulation passage and the cooling water branch passage (A'), wherein upon establishment of a first condition, the flow path switching means switches the flow path of the cooling water to supply to the thermal source device all of the cooling water cooled by the cooling tower, and upon establishment of a second condition, the flow path switching means switches the flow path of the cooling water to (i) supply the portion of the cooling water cooled by the cooling tower to the second heat exchanger, and (ii) supply to the thermal source device a portion of the cooling water other than the portion of the cooling water cooled by the cooling tower. a cooling tower configured to cool cooling water; a thermal source device configured to generate cold water by the cooling water cooled by the cooling tower; a cooling water circulation passage configured to allow circulation of the cooling water between the cooling tower and the thermal source device; a first heat exchanger configured to exchange heat between air and the cold water generated by the thermal source device; a cold water circulation passage configured to allow circulation of the cold water between the thermal source device and the first heat exchanger; a second heat exchanger configured to exchange heat between a portion of the cooling water cooled by the cooling tower and the air heat-exchanged with the cold water by the first heat exchanger; the air conditioning system characterized by further comprising a cooling water branch passage (A'), branching from the cooling water circulation passage, configured to (i) introduce to the second heat exchanger the portion of the cooling water introduced to the thermal source device from the cooling tower, and (ii) introduce to the cooling water circulation passage the portion of the cooling water heat-exchanged with the air by the second heat exchanger; and flow path switching means configured to switch a flow path of the cooling water in the cooling water circulation passage and the cooling water branch passage (A'), wherein upon establishment of a first condition, the flow path switching means switches the flow path of the cooling water to supply to the thermal source device all of the cooling water cooled by the cooling tower, and upon establishment of a second condition, the flow path switching means switches the flow path of the cooling water to (i) supply the portion of the cooling water cooled by the cooling tower to the second heat exchanger, and (ii) supply to the thermal source device a portion of the cooling water other than the portion of the cooling water cooled by the cooling tower.