Abstract: In a gas sensor, a casing which accommodates a sensing element includes an inner cylindrical member and an outer cylindrical member which are caulked together by means of a caulked portion. The Vickers hardness H1 of the inner cylindrical member is Hv 250 to 430; the Vickers hardness H2 of the outer cylindrical member is Hv 160 to 330; and the hardness difference “H1-H2” is not less than 30. Through employment of the above ranges of H1 and H2, the caulked portion can maintain good airtightness even in a working environment involving, for example, thermal shocks.

Abstract: An oxygen sensor is constituted by a ceramic separator being placed so that the rear thereof enters the inside of a filter holding part and the front enters the inside of a casing and formed with a plurality of lead insertion holes axially penetrating the ceramic separator and an elastic seal member being fitted elastically into a rear opening of the filter holding part and having seal lead insertion holes for inserting leads for sealing the gap between the outer faces of the leads and the inner face of the filter holding part. The rear end face of the ceramic separator is positioned on the rear side behind a gas introduction hole in the axial direction and a predetermined gap is formed between the elastic seal member and the ceramic separator at least at the lead insertion position.

Abstract: An oxygen sensor is constituted by a ceramic separator being placed so that the rear thereof enters the inside of a filter holding part and the front enters the inside of a casing and formed with a plurality of lead insertion holes axially penetrating the ceramic separator and an elastic seal member being fitted elastically into a rear opening of the filter holding part and having seal lead insertion holes for inserting leads for sealing the gap between the outer faces of the leads and the inner face of the filter holding part. The rear end face of the ceramic separator is positioned on the rear side behind a gas introduction hole in the axial direction and a predetermined gap is formed between the elastic seal member and the ceramic separator at least at the lead insertion position.