PURPOSE: To improve the reliability and to realize sufficiently practical use in terms of economy by providing a gas concentration switch with plural operation set values which differ stepwise and monitoring the operation time between respective set values by a judgement part.

CONSTITUTION: The gas concentration switch 5 is provided with plural working pressure setting microswitches 8, which are connected to an arithmetic processing judgement part 17 provided in a field console panel 2, so that a judgement including the operation time between respective set values is made. The output signal of the judgement part 17 is sent to a trouble alarm 9 to make an alarm control output display and also sent to a monitor station provided at distance. Thus, the switches 8 which have the stepwise set values are provided to detect the operation at the respective set values stepwise and the judgement part 17 makes overall judgements including respective operation times, so the quantity of gas leak and a gas pressure loss rate are easily grasped.