A gas sensor element comprising: an element main body including an oxygen ion-conductive solid electrolyte layer (1, 2, 3, 4, 5, 6); and a protective layer covering at least part of the element main body, the protective layer containing constituent particles, the protective layer optionally being formed of a plurality of layers; characterized in that the constituent particles have a shape in which the size in a thickness direction is smaller than the size in a direction perpendicular to the thickness direction, wherein the value of the ratio of the number of interfaces in a unit surface direction, which is the number of particle interfaces of said constituent particles every 100 μm in the surface direction perpendicular to the thickness direction, to the number of interfaces in a unit thickness direction, which is the number of particle interfaces of said constituent particles every 100 μm in the thickness direction, is more than 0 and 0.7 or less, in at least one layer when the protective layer is formed of a plurality of layers, wherein the thickness direction is a direction perpendicular to the surface of the oxygen ion-conductive solid electrolyte layer (1, 2, 3, 4, 5, 6) provided with the protective layer, wherein the number of interfaces in the unit thickness direction is determined by taking an SEM image of the protective layer, and subtracting 1 from the value obtained by counting the number of particles intersecting a base line in the thickness direction, and wherein the number of interfaces in the unit surface direction is determined by subtracting 1 from the value obtained by counting the number of particles intersecting a straight line in the surface direction perpendicular to the base line in the thickness direction in the SEM image of the protective layer. an element main body including an oxygen ion-conductive solid electrolyte layer (1, 2, 3, 4, 5, 6); and a protective layer covering at least part of the element main body, the protective layer containing constituent particles, the protective layer optionally being formed of a plurality of layers; the constituent particles have a shape in which the size in a thickness direction is smaller than the size in a direction perpendicular to the thickness direction, wherein the value of the ratio of the number of interfaces in a unit surface direction, which is the number of particle interfaces of said constituent particles every 100 μm in the surface direction perpendicular to the thickness direction, to the number of interfaces in a unit thickness direction, which is the number of particle interfaces of said constituent particles every 100 μm in the thickness direction, is more than 0 and 0.7 or less, in at least one layer when the protective layer is formed of a plurality of layers, wherein the thickness direction is a direction perpendicular to the surface of the oxygen ion-conductive solid electrolyte layer (1, 2, 3, 4, 5, 6) provided with the protective layer, wherein the number of interfaces in the unit thickness direction is determined by taking an SEM image of the protective layer, and subtracting 1 from the value obtained by counting the number of particles intersecting a base line in the thickness direction, and wherein the number of interfaces in the unit surface direction is determined by subtracting 1 from the value obtained by counting the number of particles intersecting a straight line in the surface direction perpendicular to the base line in the thickness direction in the SEM image of the protective layer.