To provide a gas detecting circuit in which the concentration of a gas being detected can be determined over a wide range, from a low concentration region to a high concentration region, using a semiconductor type gas detecting element.

In a gas detecting circuit where a gas detection voltage is applied to a semiconductor type gas detecting element 1 having a sensitive layer 11 principally comprising a metal oxide semiconductor and a gas detection output is obtained based on the variation of resistance when a gas being detected touches the semiconductor type gas detecting element 1, a constant voltage circuit part 3 for sustaining the gas detection voltage being applied to the gas detecting element 1 at a constant level is provided.