It is also possible to reset the continued period of time that has been measured when the control of increasing/decreasing supplied air volume Fsa is conducted and start the new measurement in the third technology of the second example. In other words, when for example a state in which differential pressure value ΔP is smaller than target value β has continued for m minutes, supplied air volume Fsa is increased, while when the state in which differential pressure value ΔP is smaller than target value β also continues after that, it is checked whether that state continues again for m minutes by measuring the elapsed time, and supplied air volume Fsa is further increased when it has continued for m minutes. FIGS. 5A and 5B illustrate an example in which this control increased supplied air volume Fsa and thereafter further increased it after m minutes.

From what has been described above, it is also possible to consider that the control device 13 includes the respective processing function units illustrated in for example FIG. 6. FIG. 6 is a functional block diagram of the control device 13.

In other words, the control device 13 includes a supplied air temperature control unit 31 that controls the temperature of the cold air (supplied air temperature Tsa) so that it is equal to prescribed value ST_SA and an air volume control unit 32 that adjusts and controls the air volume of the supply fan 12 so that the temperature of the return air is equal to target value ST_RA that is set in advance.