described with reference to the accompanying drawings however the prevent invention should not be construed as being limited thereto fig1 shows a schematic construction of the oxygen sensor according to one embodiment of the gas sensor of the invention as shown in fig1 an oxygen sensor 1 is provided with a detecting element 2 as a gas detecting element a heater 3 a metal shell 5 and other components of these the heater 3 is made from a rod shaped ceramic heater and is inserted into the detecting element 2 the metal shell 5 is equipped in its outer circumference with a thread 51 for mounting the oxygen sensor 1 on the mounting portion of an exhaust pipe or the like and with a bulge 53 to which a mounting fitting is applied when the sensor is mounted on the mounting portion of the exhaust pipe here the bulge 53 is equipped with a gasket 7 on its leading end face the metal shell 5 is equipped on its inner circumference with a fitting side step 54 which is radially reduced toward the leading end side interposed between the metal shell 5 and the detecting element 2 moreover are insulators 8 and 9 made from insulating ceramics a powder layer 10 of talc or the like is sandwiched under compression between insulators 8 and 9 this powder layer 10 seals the gap between the detecting element 2 and the metal shell 5 to retain gas tightness a packing 55 is sandwiched between the leading end of the insulator 9 and the fitting side.