MP-4 combustible gas sensor (gas sensor, natural gas sensor) adopts multi-layer thick film manufacturing process. The heater and metal oxide semiconductor gas sensing layer are respectively made on both sides of the micro Al2O3 ceramic substrate, and are packaged in a metal shell. The conductivity of the sensor changes when the detected gas is present in the ambient air. The higher the concentration of this gas, the higher the conductivity of the sensor. This change in conductivity can be converted into an output signal corresponding to the gas concentration using a simple circuit.