A sensor includes a pump cell and a reference cell generating voltage Vs or current. The pump cell includes a pump cell first electrode and a pump cell second electrode. The surface of the pump cell first electrode includes a noble metal region formed of a noble metal, a ceramic region, and a coexistence region in which the noble metal and the ceramic material coexist. The width of the coexistence region in an A region of the surface of the pump cell first electrode is greater than the width of the coexistence region in a B region of the surface. The A region is a region close to the reference cell first electrode, and the B region is a region located further away from the reference cell first electrode as compared with the A region.