[Objective] To provide a gas sensor in which a filter member provided in an aerial communication hole adapted to introduce the air for exposing a reference electrode of a detection element to the air is reliably protected through simple configuration.[Means for Solution] A filter member 87 having air permeability and waterproofness is disposed in an aerial communication hole 91 formed in a grommet 9 disposed in a rear end portion 38 of a housing tube 3. The housing tube 3 has a vent portion 34 provided for ensuring aerial communication while protecting the filter member 87. The vent portion 34 is connected, by means of arm portions 33, to an end portion (an opening end 32) of a side wall 39 of the housing tube 3. The side wall 39 surrounds the circumference of the grommet 9. Since the arm portions 33 are disposed in respective grooves 93 of the grommet 9, the grommet 9 is positioned relative to the housing tube 3. Thus, the vent portion 34 formed integral with the housing tube 3 is free from offset, inclination, torsion, etc., relative to the aerial communication hole 91.