A sensor element is used to detect the concentration of a predetermined component in a gas. The sensor element includes a sensor element body including a solid-state-electrolyte layer having oxygen-ion conductivity, an outer pump electrode that is disposed on an upper surface, which is one of the surfaces, of the sensor element body, and a porous protective layer that is provided so as to cover at least the outer pump electrode. A spatial layer is provided between the porous protective layer and the sensor element body. The spatial layer includes a first spatial layer between the porous protective layer and the outer pump electrode. The maximum-height roughness Rz of a region of the inner surface of the porous protective layer, the region facing the outer pump electrode, is 50 μm or smaller.