A gas detecting method includes heating a gas detector, formed of a gas sensing layer and an adsorption layer, for an oxygen adsorption period t0 at an oxygen adsorption temperature T0; heating the gas detector for a target gas adsorption period t1 at a target gas adsorption temperature T1 at which a target gas is adsorbed to the adsorption layer, heating the gas detector for a target gas desorption period t2 at a target gas desorption temperature T2 at which the target gas adsorbed to the adsorption layer is desorbed to move to the gas sensing layer; and calculating a gas concentration of the target gas from a sensor resistance of the gas sensing layer. A gas detecting device includes the gas detector and a drive processing unit configured to carry out the method.