PURPOSE: To estimate the leak spot and quantity of gas by calculating the gas flux of leaked gas proportional to the leak quantity of gas from wind direction and velocity data and leaked gas concn. data.

CONSTITUTION: A gas concn. data collecting and storing device 1 and a wind direction and velocity data collecting and storing device 2 input the measured data of (M) gas detectors G and (N) aerovanes B (M≥2, N≥1, M>N>M/20) to collect and store those data while renew them. A data processor 3 multiplies the gas concn. data and wind direction and velocity data from the devices 1, 2 to calculate gas flux and a CRT display device 8 sets the magnitude of the gas flux to a length vector and a wind direction to a direction vector to display the gas flux. Two selected gas detectors are extended so as to permit the correction vectors to pass to calculate lines and the crossing point of those lines is calculated. When the leak quantities of gas operated by the processor 3 coincide with each other, the crossing point is judged to be a real gas leak spot and the gas leak quantity at the leak spot.