A gas sensor (1) has a platelike detection element (5) extending in an axial direction and a ceramic member (67) having an element insertion hole (81) into which the detection element (5) is inserted. When the ceramic member (67) is viewed from the axial direction, the element insertion hole (81) has a main insertion hole (83) having a substantially quadrate shape surrounded by four sides, and relief holes (85), (86), (87) and (88), each of which is surrounded by an outline connecting ends of two adjacent sides of the four sides of the main insertion hole (83) and located radially outward of the main insertion hole (83) so as to communicate with the main insertion hole 83.