<p>The disclosed gas sensor is provided with an electric-potential-applying device-which is provided with: an electrolyte; a working electrode and counter electrode that sandwich the aforementioned electrolyte; and an ammeter that measures the current flowing between the aforementioned working electrode and the aforementioned counter electrode-that detects the density of the gas to be measured in a measured gas by means of the value of the current flowing between both the aforementioned electrodes, and applies an electric potential to the aforementioned working electrode that is the same value as the equilibrium potential of the electrochemical reaction that the impurity gas within the measured gas participates in, or an electric potential in a range of ±20 mV of the aforementioned equilibrium potential.</p>
