To provide a simple-structure air conditioning system capable of using heat source equipment on the market for air conditioning equipment having level difference at a maximum use pressure or less.

This air conditioner mainly comprises a cold heat or cold warm heat source machine 1, an air conditioner 4, a cooling tower 7, a cooling or heating medium pump 5. This air conditioning system is provided with constant pressure retaining tanks 13A and 13B in a cooling or heating medium, or heat radiating medium flow passage with a pressure difference for retaining the pressure of the maximum pressure part by the cooling or heating medium, or the heat radiating medium of the machine 1 to low. The constant pressure retaining tanks 13A and 13B are preferably provided in an elevated place in the medium flow passage with the pressure difference as high as possible, and the machine 1 uses the cooling water for the heat radiating medium, is provided with equipment radiating the heat by the cooling water 7. In this case, if the machine 1 is a compression type, the maximum pressure part of the heat radiating medium is a condenser, while if it 1 is an absorption type, the maximum pressure part of the heat radiating medium is an absorber or the condenser.