PURPOSE: To improve the data transmission efficiency of the title device by providing a means which sets a data bit number in corresponding to the kind of the sensor of terminal equipment to a central monitor and a means which returns data of a prescribed bit number in response to a data returning request called from the central monitor to the terminal equipment.

CONSTITUTION: The central monitor 1 sets the bit number of sensor data to each terminal equipment 3 from kind data from each terminal equipment 3 and does not perform fetching of returned data unless the returned data having the bit number corresponding to each terminal equipment 3 when an ordinary data returning request is made. In addition, each terminal equipment 3 is constituted in such a way that the equipment 3 returns only the returning data of the bit number set to its own equipment upon receiving a call of data returning request. In other words, the length of the returning data is different depending upon the kind of each terminal equipment 3 and the returning data can be returned without vacant space. The central monitor 1 performs judging processing in accordance with the kind of each terminal equipment 3 from the returned data and a warning display or control of an air-conditioning device depending upon a judged result. Therefore, the data transmission efficiency of this device can be improved as a whole and the transmission speed is apparently improved.