A gas sensor (100) includes an oxygen pump cell (135) and an oxygen-concentration detection cell (150) laminated together with a spacer (145) interposed therebetween. The spacer (145) has a gas detection chamber (145c) which faces electrodes (137, 152) of the cells (135, 150). The oxygen-concentration detection cell (150) produces an output voltage corresponding to the concentration of oxygen in the gas detection chamber (145c). The oxygen pump cell (135) pumps oxygen into and out of the measurement chamber (145c) such that the output voltage of the oxygen-concentration detection cell (150) becomes equal to a predetermined target voltage. A leakage portion mainly formed of zirconia is disposed between which electrically connects the oxygen-concentration detection cell (150) and the oxygen pump cell (135).