To provide a long-life gas concentration detector having an elastic insulation member, which has a good sealing property between a lead wire and a lead wire insertion hole, and has a high heat resistance for a long term.

The oxygen concentration detector 1 comprises a sensing element, protective covers 11 and 12, lead wires 31 and 32, and an elastic insulation member 2. The sensing element is inserted and arranged in a housing. The protective covers 11 and 12 are arranged on the upper part of the housing. The lead wires 31 and 32 are inserted to the protective covers 11 and 12. The elastic insulation member 2 is arranged within the protective covers 11 and 12 and has a lead wire insertion hole 20 for insertion of the lead wires 31 and 32. The elastic insulation member 2 is caulked to be fixed by the protective covers 11 and 12. The elastic insulation member 2 is made from polytetrafluoroethylene and a density of the elastic insulation member 2 is 1.7-2.0 g/cm3 in a free state in which the elastic insulation member 2 is not caulked and fixed.