Abstract: Outer air serving as a reference gas for determining oxygen concentration is guided to a hollow portion of a core. The core is formed in an insulating cylinder whose distal end is closed and whose proximal end is open to receive the reference gas. An opening is formed on the outer surface of the core, to communicate with the hollow portion. An oxygen concentration sensor element made of a solid-state electrolyte layer having first and second electrode layers one on each surface thereof is mounted such that it closes the opening. A sheet member made of an insulating sheet is wound around the sensor element mounted on the sensor element, and is fixed thereon. A window is formed in the sheet so as to expose the surface of the sensor element. A heater is formed around the window. Wiring layers respectively connected to both terminals of the heater are formed on the sheet. Wiring layers adapted to contact the first and second electrode layers of the sensor element are also formed on the sheet.