A mixed-potential gas sensor that includes a sensor element (101, 151) constituted by an oxygen-ion conductive solid electrolyte (4, 5, 6) and measures a concentration of a predetermined gas component of a measurement gas, said sensor element comprising: a first gas inlet communicating with the outside; a first internal space communicating with said first gas inlet via a predetermined diffusion control part; a second internal space communicating with said first internal space via another predetermined diffusion control part; a second gas inlet (41A, 41B) causing said outside and said second internal space to communicate with each other; a sensing electrode located in said second internal space; a reference gas introduction space (51, 52) into which a reference gas is introduced; a reference electrode located in said reference gas introduction space, said solid electrolyte being located between said sensing electrode and said reference electrode; an oxygen pumping cell including an inside pump electrode facing said first internal space, an outside pump electrode located on an outside surface of said sensor element, and said solid electrolyte located between said inside pump electrode and said outside pump electrode; and a main gas distribution part being a path extending from said first gas inlet to said second internal space; wherein said sensor element further comprises, in said second internal space, a monitoring electrode for monitoring the oxygen concentration of said second internal space, and said oxygen pumping cell is arranged to pump oxygen in or out for said measurement gas flowing via said main gas distribution part on the basis of the potential difference between said monitoring electrode and said reference electrode, said gas sensor is configured and disposed so as to enable measurement of the concentration of said predetermined gas component contained in the measurement gas introduced from said second gas inlet (41A, 41B) into said second internal space on the basis of a potential difference between said sensing electrode and said reference electrode, while said oxygen pumping cell pumps oxygen in or out of said measurement gas flowing via said main gas distribution part such that the oxygen concentration of said second internal space is maintained at a constant value equal to or more than 1 vol%, and 3.5 ≤ D 2 / D 1 ≤ 6 is satisfied, where D1 is a value of a diffusion resistance of said measurement gas flowing from an outside edge position of said first gas inlet to said sensing electrode via said main gas distribution part, and D2 is a value of a diffusion resistance of said measurement gas flowing from an outside edge position of said second gas inlet to said sensing electrode. a first gas inlet communicating with the outside; a first internal space communicating with said first gas inlet via a predetermined diffusion control part; a second internal space communicating with said first internal space via another predetermined diffusion control part; a second gas inlet (41A, 41B) causing said outside and said second internal space to communicate with each other; a sensing electrode located in said second internal space; a reference gas introduction space (51, 52) into which a reference gas is introduced; a reference electrode located in said reference gas introduction space, said solid electrolyte being located between said sensing electrode and said reference electrode; an oxygen pumping cell including an inside pump electrode facing said first internal space, an outside pump electrode located on an outside surface of said sensor element, and said solid electrolyte located between said inside pump electrode and said outside pump electrode; and a main gas distribution part being a path extending from said first gas inlet to said second internal space; wherein said sensor element further comprises, in said second internal space, a monitoring electrode for monitoring the oxygen concentration of said second internal space, and said oxygen pumping cell is arranged to pump oxygen in or out for said measurement gas flowing via said main gas distribution part on the basis of the potential difference between said monitoring electrode and said reference electrode, said gas sensor is configured and disposed so as to enable measurement of the concentration of said predetermined gas component contained in the measurement gas introduced from said second gas inlet (41A, 41B) into said second internal space on the basis of a potential difference between said sensing electrode and said reference electrode, while said oxygen pumping cell pumps oxygen in or out of said measurement gas flowing via said main gas distribution part such that the oxygen concentration of said second internal space is maintained at a constant value equal to or more than 1 vol%, and 3.5 ≤ D 2 / D 1 ≤ 6 is satisfied, where D1 is a value of a diffusion resistance of said measurement gas flowing from an outside edge position of said first gas inlet to said sensing electrode via said main gas distribution part, and D2 is a value of a diffusion resistance of said measurement gas flowing from an outside edge position of said second gas inlet to said sensing electrode.