GAS DETECTOR AND AUTOMOBILE VENTILATION SYSTEM

It is an object of the invention to provide a gas detecting device capable of relieving the influence of an environment such as a temperature, a humidity or a wind velocity to detect a change in a concentration of a specific gas and an autoventilation system for a vehicle using the gas detecting device. More specifically, a gas detecting device ( 10 ) using a gas sensor element ( 11 ) changing a sensor resistance Rs depending on a concentration of a gas such as NOx and an autoventilation system ( 100 ) for a vehicle according to the invention A/D convert the output of a sensor resistance value converting circuit ( 14 ) to acquire a sensor output value S(n), calculates a base value B(n) based on B(n)=B(n-1)+k 1 {S(n)-B(n-1)}, and furthermore, calculates a difference value of D(n)=S(n)-B(n). When the concentration of the gas is increased so that the difference value becomes greater than a predetermined concentration threshold, a high concentration signal is generated to close a flap ( 34 ) and a base value B(n) is calculated by using a coefficient k 2 which is smaller than k 1 based on B(n)=B(n-1)+k 2 {S(n)-B(n-1)}.