A gas sensor element includes a base member comprising a plurality of laminated solid electrolyte layers, and having a space that communicates with the outside of the gas sensor element and allows introduction of the gas to be measured into the gas sensor element, and a porous measurement electrode that is formed on a surface of the space inside the base member, and is covered with a porous measurement electrode protective layer, wherein an average pore size A of the measurement electrode and an average pore size B of the measurement electrode protective layer satisfy the relationship “0.05≦B/A≦0.9”, the measurement electrode has an average pore size of 0.5 to 15 μm, and the measurement electrode protective layer has an average pore size of 0.05 to 9 μm, a porosity of 5 to 50%, and a thickness of 10 to 200 μm.