PURPOSE: To quickly change the generated heat output of a cold and hot water generating part in response to the change of the thermal demand on the side to be air-conditioned by a method wherein the cold and hot water generating capacity of the cold and hot water generating part is determined based upon the heat load factor calculated from the change of temperature in the space to be air-conditioned.

CONSTITUTION: The gradient of change of room temperature detected by the room temperature detecting end of a room temperature conditioner 12a is calculated at a signal processing unit 20a or secondary side (side to be air-conditioned) so as to judge that the calculated inclination corresponds to which of the inclinations of lowering of room temperature during a certain period of time in order to estimate the load factor of a space to be air-conditioned B1 and the like. When the load factor is estimated at 100%, the output on primary side (of the cold and hot water generating part) needs not be changed. When the load factor is estimated at less than 100%, the number of cold and hot water generators 2aW2d in operation is reduced properly. On the contrary, when the load factor is estimated at more than 100%, the number of the cold and hot water generators in operation is increased.