A sensor element includes: a main pump cell constituted by an inner pump electrode facing a first inner space into which a measurement gas is introduced, an external pump electrode provided on an element surface, and a solid electrolyte located therebetween; and a measurement pump cell constituted by the measurement electrode facing a second inner space which is communicated with the first inner space and functioning as a reduction catalyst for NOx, and a solid electrolyte located therebetween. The inner pump electrode has a planar shape in which two parts of a front end part relatively having a large area and a rear end part relatively having a small area are sequentially connected in this order from an upstream side in a longitudinal direction of the sensor element while satisfying requirements of a predetermined size and area.