A gas sensor, which is extremely compact to be arranged for separated gas piping in semiconductor device manufacturing equipment, a gas measuring system using such gas sensor, and a gas detection module for the gas measuring system. The gas sensor has a gas detection device containing a dielectric semiconductor, the electric conductivity of the gas detection device varying in response to the degree of adsorption of gases to the gas detection device, a capacitive element connected in series to the gas detection device, and a pair of electrodes which are connected to electric terminals of an electric element comprising the gas detection device and the capacitive element, wherein the gas sensor is capable of detecting the degree of adsorption of gases to the gas detection device from an electrical response to a voltage which is applied to the electrodes and which periodically varies and reverses in polarity.