A gas sensor element includes a basal body, at least one solid electrolyte portion and a pair of electrodes. The basal body has a bottomed tubular shape and is made of an electrically insulative ceramic material. The basal body has a side wall and a bottom wall. The at least one solid electrolyte portion is formed in the bottom wall or the side wall of the basal body. The pair of electrodes are opposed to each other with the at least one solid electrolyte portion interposed therebetween. The difference in surface level between the basal body and the at least one solid electrolyte portion at a boundary therebetween is less than or equal to 30 m.