This invention provides a high-reliability gas-sensor-element assembly that, even if detents consisting of planar notches are provided in sections of the outer surface of a tubular insulator that contains and secures a plate-shaped sensor element that has a rectangular cross section and extends in an axial direction and even if an outside glass diameter is made as small as possible, improves contact between the insulator and sealing glass while minimizing reductions in the mechanical strength of the insulator. By making it such that the inner surface of a glass-filled space (32) that is filled with sealing glass (2) provided inside an insulator (3) provided with detents (33) has an oval cross section or a rounded-corner polygonal cross section with rounded corners, the sealing glass (2) is made to have an oval cross section or a rounded-corner polygonal cross section with curved corners, the opposite-corner-position thickness (T20) of the sealing glass (2) along the extension of a line that connects opposite
