A sensor element comprising: a laminate, in which a plurality of oxygen ion-conductive solid electrolyte layers (1, 2, 3, 4, 5, 6) are stacked and in which a measurement object-gas flow portion (10, 11, 12, 13, 20, 30, 40) is disposed so as to introduce the gas to be measured from the outside; a porous cermet outside pump electrode disposed on the outside surface of the laminate; a porous cermet inside main pump electrode disposed on the inner circumferential surface of the measurement object-gas flow portion; a porous cermet inside auxiliary pump electrode disposed on the inner circumferential surface of the measurement object-gas flow portion and disposed on the side downstream from the inside main pump electrode in the flow of the gas to be measured; a porous cermet measurement electrode disposed on the inner circumferential surface of the measurement object-gas flow portion and disposed on the side downstream from the inside auxiliary pump electrode in the flow of the gas to be measured; and a reference electrode which is disposed in the laminate and to which a reference gas serving as the reference of detection of a specific gas concentration in the gas to be measured is introduced, characterized in that the value of the volume ratio Va is 14.5 or more and 51.5 or less and the value of the volume ratio Vb is 5.0 or more and 17.5 or less, where the volume ratios Va = Vm/Vr, Vb = Vs/Vr and the volume of the inside main pump electrode is specified as Vm, the volume of the inside auxiliary pump electrode is specified as Vs, and the volume of the measurement electrode is specified as Vr. a laminate, in which a plurality of oxygen ion-conductive solid electrolyte layers (1, 2, 3, 4, 5, 6) are stacked and in which a measurement object-gas flow portion (10, 11, 12, 13, 20, 30, 40) is disposed so as to introduce the gas to be measured from the outside; a porous cermet outside pump electrode disposed on the outside surface of the laminate; a porous cermet inside main pump electrode disposed on the inner circumferential surface of the measurement object-gas flow portion; a porous cermet inside auxiliary pump electrode disposed on the inner circumferential surface of the measurement object-gas flow portion and disposed on the side downstream from the inside main pump electrode in the flow of the gas to be measured; a porous cermet measurement electrode disposed on the inner circumferential surface of the measurement object-gas flow portion and disposed on the side downstream from the inside auxiliary pump electrode in the flow of the gas to be measured; and a reference electrode which is disposed in the laminate and to which a reference gas serving as the reference of detection of a specific gas concentration in the gas to be measured is introduced, characterized in that the value of the volume ratio Va is 14.5 or more and 51.5 or less and the value of the volume ratio Vb is 5.0 or more and 17.5 or less, where the volume ratios Va = Vm/Vr, Vb = Vs/Vr and the volume of the inside main pump electrode is specified as Vm, the volume of the inside auxiliary pump electrode is specified as Vs, and the volume of the measurement electrode is specified as Vr.