PROBLEM TO BE SOLVED: To provide a water-cooling type air conditioning system that is preferable for medium or small sized buildings.SOLUTION: A circulation amount of water attained by a secondary side circulation pump 3g is decreased due to a reduction of cooling demand of an air conditioner 4. In turn, since a circulation amount of water attained by a primary side circulation pump 2b does not change, a difference in the water amount is supplied to a bypass pipe 2g and its corresponding cold heat is accumulated at an accumulated or radiated heat device 5 in the bypass pipe 2g. In the case that the inter-head differential pressure is decreased lower than a predetermined minimum value from this state, it is determined that the cooling load at the air conditioner is further low, a heat source machine 2a and the primary side circulation pump 2b are stopped and a two-way valve 2h is closed. With this operation, the cooling operation of the air conditioner 4 is carried out through radiation from the accumulated or
