In an air conditioning system comprising a heat source side machine for condensing a fluid which can change a phase between a liquid phase and a gas phase to a predetermined temperature and for supplying, and a plurality of user side machines more than half of which are disposed below said heat source side machine, in which piping is constructed in such a manner as to circulate the fluid supplied from said heat source side machine by using a difference in specific gravity between the liquid phase and the gas phase, between said heat source side machine and said user side machine and a cooling of said user side machine is performed by evaporating said fluid in said user side machine, the heat source side machine is provided with a control means for descending a set temperature of said fluid discharged from said heat source side machine after condensed by changing the operating capacity of said heat source side machine when a state that the temperature of said fluid returned back after evaporated in the user side machine at a time of a cooling is higher than a predetermined temperature continues for a predetermined time period. Accordingly, even when the cooling effect is insufficient due to the super-heated state of the fluid in the user side machine, the super-heated state of the fluid is resolved after the predetermined time has passed, thereby returning to the normal cooling