To detect gas in a real time in a plurality of measuring points, to transmit a measured data in a real time, and to monitor and control centralizingly the measured data in the plurality of measuring points.

The gases from the plurality of measuring points 10, 20, 30 in a remote site are transferred to detectors by gas detectors 110, 120, a piping switching device 130 and a distributor 140 in an APCI-MS (atmospheric pressure chemical ionization mass spectrometer), and the gasses are detected in the real time. The data obtained in the detectors are transmitted to a communication control computer 103 and a remote monitoring station 150 via a communication line, so as to allow the centralized monitoring and control in the remote site.