A gas sensor element includes: an element base being a ceramic structure including a sensing part; and a leading-end protective layer being a porous layer disposed around an outer periphery of the element base in a predetermined range on a side of the sensing part. The leading-end protective layer includes: a first layer disposed at least on two main surfaces of the element base; a second layer disposed to cover the end portion and four side surfaces of the element base including the two main surfaces; and a third layer disposed to cover the second layer. The second layer has a porosity of 30% to 80%, and has a thickness of 30 to 50 times thickness of the first layer, and the third layer has a porosity of 15% to 30%, and has a thickness of 5 to 10 times the thickness of the first layer.