In a sensor element 101, oxygen contained in a gas to be measured introduced into a first internal space 20 through a first diffusion control part 11 is pumped out by applying voltage between an inner pump electrode 22 and an outer pump electrode 23. After the oxygen is pumped out, NOx in the gas to be measured generates oxygen by being reduced by a measurement electrode 44. This oxygen is pumped by applying voltage between the measurement electrode 44 and the outer pump electrode 23. On the basis of current generated according to the amount of oxygen thus pumped, the NOx gas concentration is calculated. A slit width of the first diffusion control part 11 on an entrance side is larger than a slit width on an exit side.