Oxygen sensor

An oxygen sensor (1) comprising: an oxygen sensing element (2) shaped like a shaft; a cylindrical casing (10) for housing said oxygen sensing element (2); a gas introduction structure (16) having a filter holding part (51) making a cylindrical form almost coaxial with said casing (10) on a rear of said casing (10), having an inside communicating with an inside of said casing (10), and being formed in a wall with one or more gas introduction holes (52) and a filter (53) being placed so as to block the gas introduction hole or holes (52) of the filter holding part (51) for rejecting permeation of liquid and allowing gas to pass through, said gas introduction structure (16) for introducing outside air into said casing (10) through the filter (53) and the gas introduction hole or holes (52); a ceramic separator (18) being placed so that a rear thereof enters the inside of the filter holding part (51) in an axial direction of said oxygen sensing element (2) and a front enters the inside of said casing (10) and formed with a plurality of lead insertion holes (72) axially penetrating said ceramic separator (18) for inserting leads (20, 21, 28, 29) from said oxygen sensing element (2); and an elastic seal member (17) being fitted elastically into a rear opening of the filter holding part (51) and having seal lead insertion holes (91) for inserting the leads (20, 21, 28, 29) for sealing a gap between outer faces of the leads and an inner face of the filter holding part (51); wherein a rear end face of said ceramic separator (18) is positioned on the rear side behind the gas introduction hole (52) in the axial direction and a predetermined gap is formed between said elastic seal member (17) and said ceramic separator (18) at least at the lead insertion position.