A gas sensor element (10) includes a composite ceramic layer (111) having an insulation portion (112) and an electrolyte portion (131) disposed within a through hole (112h), and a first conductor layer (150) extending in a continuous manner on a first insulation main surface (113) as well as on a first electrolyte main surface (133). The electrolyte portion (131) is thinner than the insulation portion (112), and the first electrolyte main surface (133) is located on a thickness-direction inward side DTN. The insulation portion (112) has, on a first insulation main surface side, a protruding portion (122) overlying the first electrolyte main surface (133). The thickness of the protruding portion (122) reduces toward the inward side DR1 of the through hole (112h). The first conductor layer (150) extends in a continuous manner on a protrusion surface (122s) as well as on the first electrolyte main surface (133).
