A gas sensor element includes a solid electrolyte body having oxygen ion conductivity, a pair of measurement and reference electrodes respectively provided on an opposite pair of first and second surfaces of the solid electrolyte body, a porous diffusion-resistant layer through which a measurement gas is introduced to the measurement electrode, and a protective layer. The protective layer is provided to cover, at least, an outer surface of the porous diffusion-resistant layer through which the measurement gas flows into the diffusion-resistant layer. The protective layer is hydrophilic at room temperature and water-repellent at high temperatures at which the solid electrolyte body can be activated.