NOVELTY - The gas sensor includes a second oxygen pump (14) comprising a second inner electrode (10) and a second outer electrode (11) formed on an oxygen-ion conductor (1) so as to face the interior and exterior, of the second processing chamber (5) and adapted to decompose into oxygen ions O2 which remains unconsumed in reaction with combustible gas components in the second processing chamber (5), through application of a constant voltage between the second inner electrode (10) and the second outer electrode (11) and to pump out the generated oxygen ions from the second processing chamber (5) though the oxygen-ion conductor.    USE - Used for measuring the concentration of a combustible gas component.    ADVANTAGE - Provides simple structure and capable of measuring combustible gas components at high accuracy and over a wide range, as well as to provide a measuring apparatus using the gas sensor.    DETAILED DESCRIPTION - A gas sensor comprises (a) a first processing chamber (3) and a first gas passage (2) for introducing a gas under measurement into the first processing chamber (3); (b) a second processing chamber (5) and a second gas passage (4) for introducing the gas from the first processing chamber (3) into the second processing chamber (5), the second processing chamber (5) being provided for the reaction of combustible gas components contained in the gas with O2; (c) a third gas passage (15;16) for introducing a gas containing O2 into the second processing chamber (5) under diffusion resistance; (d) an oxygen -concentration sensor (13) for sensing the oxygen concentration gas introduced from the first processing chamber (3) to the second processing chamber (5); (e) a first oxygen pump (12) comprising a first inner electrode (7) and a first outer electrode (6) formed on an oxygen-ion conductor (1) so as to face the interior and exterior, respectively, of the firs t processing chamber (3), and adapted to decompose O2 into oxygen ions through application of voltage between the first inner electrode (7) and the first outer electrode (6) on the basis of an oxygen-concentration sensor (1) and to pump out the generated oxygen ions from or pump into the first processing chamber (3) through the oxygen-ion conductor to control the oxygen concentration of the gas introduced from the first processing chamber(3) to the second processing chamber; (f) device for accelerating the reaction of O2 and combustible gas contained in the gas introduced into the second processing chamber and (g) a second oxygen pump (14). An INDEPENDENT CLAIM is included for an apparatus for measuring combustible gas component concentration.    DESCRIPTION OF DRAWING(S) - The diagram shows a lateral view of the sensor.    first and second gas passages (2,4)    first and second processing chambers (3,5)    third gas passage (15;16)    oxygen-concentration sensor (13)    sensor (36)    first and second oxygen pumps (12,14)    first outer and inner electrodes (6,7)    oxygen-ion conductor (1)