Air/fuel ratio detection apparatus

An air/fuel ratio detection apparatus for a gas, comprising:

• a heater portion that has an elongate cylindrical shape and heats by applying electricity to the heater portion from an outside;
• a solid electrolyte layer formed on and entirely surrounding periphery of the heater portion, the solid electrolyte layer being activated by heat from the heater portion to conduct oxygen ions through the solid electrolyte layer;
• a first electrode that is positioned between the heater portion and the solid electrolyte layer and is in contact with an inner surface of the solid electrolyte layer;
• second and third electrodes that are in contact with an outer surface of the solid electrolyte layer and are away from each other such that the solid electrolyte layer is interposed between the first and second electrodes and between the first and third electrodes;
• a first voltage applying device for applying a first voltage between the first and second electrodes;
• and
• a second voltage applying device for applying a second voltage between the first and third electrodes, the second voltage being higher than the first voltage,
• wherein the first and second voltage applying devices are configured such that the first and second voltages are adjusted to allow an output signal from the second electrode to change according to air-fuel ratio of the gas.