A gas sensor, comprising: a sensor element extending in an axial direction and having a detection portion for detecting a gas to be detected at a front end thereof; a metal shell that surrounds a periphery of the sensor element in radial direction while allowing the detection portion to protrude from a front end thereof; a cylindrical outer tube disposed at a rear side of the metal shell and having a front end fixed to the metal shell; and a seal member disposed in the outer tube, the seal member including: a lead wire insert hole into which a lead wire for extracting a detection signal of the sensor element is inserted, the lead wire insert hole extending in an axial direction thereof, and an atmosphere communication hole allowing atmospheric communication between an inside and an outside of the outer tube through a filter member having air ventilation and waterproofing properties, the atmosphere communication hole extending in an axial direction thereof, wherein the seal member has a groove extending outwards in a radial direction at a rear end-facing surface of the seal member from the atmosphere communication hole while circumventing the lead wire insert hole, the groove having a cutout toward the front end of the seal member, a bottom surface and a side surface, wherein the gas sensor further includes: a protection portion that protects the filter member by at least partially covering the atmosphere communication hole of the seal member from the rear end of the gas sensor in the axial direction; and an arm that couples the protection portion and the outer tube, and extends in radial direction, the arm being at least partially disposed in the groove of the seal member; wherein the arm is disposed so as to define a gap with the bottom surface of the groove, characterised in that the size of the gap is suitably set so that the arm does not contact the bottom of the groove. a sensor element extending in an axial direction and having a detection portion for detecting a gas to be detected at a front end thereof; a metal shell that surrounds a periphery of the sensor element in radial direction while allowing the detection portion to protrude from a front end thereof; a cylindrical outer tube disposed at a rear side of the metal shell and having a front end fixed to the metal shell; and a seal member disposed in the outer tube, the seal member including: a lead wire insert hole into which a lead wire for extracting a detection signal of the sensor element is inserted, the lead wire insert hole extending in an axial direction thereof, and an atmosphere communication hole allowing atmospheric communication between an inside and an outside of the outer tube through a filter member having air ventilation and waterproofing properties, the atmosphere communication hole extending in an axial direction thereof, a lead wire insert hole into which a lead wire for extracting a detection signal of the sensor element is inserted, the lead wire insert hole extending in an axial direction thereof, and an atmosphere communication hole allowing atmospheric communication between an inside and an outside of the outer tube through a filter member having air ventilation and waterproofing properties, the atmosphere communication hole extending in an axial direction thereof, wherein the seal member has a groove extending outwards in a radial direction at a rear end-facing surface of the seal member from the atmosphere communication hole while circumventing the lead wire insert hole, the groove having a cutout toward the front end of the seal member, a bottom surface and a side surface, wherein the gas sensor further includes: a protection portion that protects the filter member by at least partially covering the atmosphere communication hole of the seal member from the rear end of the gas sensor in the axial direction; and an arm that couples the protection portion and the outer tube, and extends in radial direction, the arm being at least partially disposed in the groove of the seal member; wherein the arm is disposed so as to define a gap with the bottom surface of the groove, characterised in that the size of the gap is suitably set so that the arm does not contact the bottom of the groove. a protection portion that protects the filter member by at least partially covering the atmosphere communication hole of the seal member from the rear end of the gas sensor in the axial direction; and an arm that couples the protection portion and the outer tube, and extends in radial direction, the arm being at least partially disposed in the groove of the seal member; wherein the arm is disposed so as to define a gap with the bottom surface of the groove, characterised in that the size of the gap is suitably set so that the arm does not contact the bottom of the groove.