To provide a manufacturing method of a gas sensor and the gas sensor capable of improving assembling workability, suppressing an influence of an error of a component size or the like, suppressing a distorted deformation, a crack or the like of a casing, and surely caulking and fixing an elastic seal member, and having excellent airtightness.

In this manufacturing method of the gas sensor 1, a compression external force F is applied to the seal member 11 to thereby compress and deform the member 11, and hereby the radial direction size of a seal member 11 can be enlarged compared with the case before compression. Hereby, since the gap between the outer face of the seal member 11 and the inner face of an external cylinder 6 (in detail, a cylindrical aperture 63) is reduced, even if a caulking deformation rate is small, a caulking fixing force of the seal member 11 by a compression maintaining caulking part 66 can be secured greatly, to thereby improve airtightness. By forming a fixing caulking part 67 in addition to the compression maintaining caulking part 66, the contact area between the cylindrical aperture 63 of the external cylinder 6 and the seal member 11 can be enlarged, and the airtightness can be improved furthermore.