A wide range A/F(air to fuel) ratio sensor was fabricated in a plate type. The sensor has two electrochemical cells and a reference room between them. The cathode of the first cell is covered by a gas diffusion barrier and exposed to the exhaust gas. The anode is exposed to the reference room. The room is supplied of a constant oxygen flow from the oxygen ionic pumping of the second cell. A porous diffuson wall is surrounding the room to prevent an excess $O_2$ pressure. So the room is maintatined in a constrant $O_2$ pressure of about the atmospheric air. The characteristic of the sensor was expected by the simulation and compared with the experimental results. It was possible to detect wide range of the A/F ratio under a constant applied voltage.