To provide a gas sensor with a sensor element prevented from suffering both water-induced cracking and responsivity lowering.

An air-fuel ratio sensor 11 includes a sensor element 40 comprising: a detection part 48 for detecting the concentration of oxygen which is a specific gas component contained in exhaust gas of an internal combustion engine; and a heater heating part provided substantially on the same place as the detection part 48 with respect to the flow of the exhaust gas for heating the detection part 48. The whole of a domain contacted by the exhaust gas out of the surface of the sensor element 40 is covered with a water-absorbing porous protective layer 60. This protective layer 60 is equipped with upstream side parts 61 and 62 formed on the upstream side of the flow of the exhaust gas relative to the detection part 48 and the heater heating part to absorb waterdrops in the exhaust gas, and a downstream side part 63 formed so as to be thinner than the side parts 61 and 62 around the detection part 48 and the heater heating part with the exhaust gas having gone through the side parts 61 and 62 guided thereto.