<p><P>PROBLEM TO BE SOLVED: To guarantee safety of an operator by measuring a gas concentration at a spot where a concentration sensor is not installed, and to realize evacuation to a safe area and danger precognition by a simple and small-scale system configuration. <P>SOLUTION: A concentration information operation holding part 11 acquires gas concentration information in a controlled area where the concentration sensor is not installed by communication with one or more other concentration information operation holding means in an allowable transmission/reception range of a radio wave, holds the mean value thereof, and informs a terminal device of the averaged gas concentration information. A dangerous region information communication part 12 predicts a future gas concentration change from the gas concentration information and information on the wind velocity and the wind direction acquired by a flow velocity sensor, and informs the terminal device of a dangerous region in the controlled area. <P>COPYRIGHT: (C)2004,JPO</p>
