A gas sensor element includes a main body and a protective layer. The main body has four plane portions and four corner portions each of which is formed between one adjacent pair of the plane portions. The four corner portions include a pair of first corner portions that are formed on a porous diffusion-resistant layer side in a lamination direction of the main body and a pair of second corner portions that are formed on a heater layer side in the lamination direction. The protective layer is comprised of an inner protective layer that covers at least the first corner portions of the main body and an outer protective layer that covers the entire outer periphery of the main body and the inner protective layer. The protective layer has a larger average thickness at the first corner portions than at the plane portions of the main body.