To obtain an air-fuel ratio sensor exhibiting a high yield of manufacture in which a stabilized pumping current can be obtained by preventing a material forming a spray layer of spinel, or the like, from adhering to a porous tubular body responsible for diffusion controlled reaction through a gas diffusion hole.

The air-fuel ratio sensor comprises a sensing section having a pair of electrodes 3 and 4 formed at the opposite positions of a solid electrolytic substrate, a solid electrolytic layer 8 formed on the outer surface of a measuring electrode 4 through a space section 5, a gas diffusion hole 11 made through the solid electrolytic layer 8, a porous tubular body 16 formed around the diffusion hole in a space section 5, and a ceramic spray layer 34 formed around the gas diffusion hole 11 on the outer surface of the solid electrolytic layer 8 wherein the inside diameter x of the gas diffusion hole 11 on the outer surface thereof is set smaller than the inside diameter y of the porous tubular body.