A gas sensor element (10) includes a first composite ceramic layer (111) having a plate-shaped first surrounding portion (112) formed of an insulating ceramic and including a through hole inner-perimetric-surface (115) which defines a through hole (112h), and a plate-shaped first electrolyte portion (121) formed of a solid electrolyte ceramic, disposed in the through hole (112h), and including an electrolyte outer-perimetric-surface (125) in contact with the through hole inner-perimetric-surface (115). The electrolyte outer-perimetric-surface (125) of the first electrolyte portion (121) is sloped toward an exterior of the first electrolyte portion (121) as it approaches one side DT1. The through hole inner-perimetric-surface (115) and the electrolyte outer-perimetric-surface 125 are in close contact with each other along their entire perimeters.