A gas sensor element (100) including a detection portion (150) including a solid electrolyte body (105) and a pair of electrodes (104) and (106) disposed on the solid electrolyte body; and a porous protection layer (20) covering the detection portion. The porous protection layer includes an inner porous layer (21) and an outer porous layer (23); the inner porous layer is higher in porosity than the outer porous layer; the inner porous layer contains, as main components, ceramic particles (21a), and ceramic fiber filaments (21b) which are mainly formed of a ceramic material and which have a mean fiber length of 70 to 200 μm; and the amount of the ceramic fiber filaments is 25 to 75 vol % on the basis of the total amount of the ceramic particles and the ceramic fiber filaments, the total amount being taken as 100 vol %.