Gas sensor

A gas sensor comprising:

• a sensor element including a solid electrolyte that has ionic conductivity and in which a tip portion of a tube having a tubular shape is blocked by a bottom having a curved surface, a detection electrode that is provided on an outer surface of the solid electrolyte and is exposed to detection gas, and a reference electrode that is provided on an inner surface of the solid electrolyte and is exposed to reference gas;
• and
• a heater that has a tip portion including a heating section for heating the solid electrolyte and is arranged inside the solid electrolyte for a tip of the tip portion to contact with the inner surface of the bottom, wherein:
• the reference electrode includes
• an inner detection section that is provided on an entire periphery in a circumferential direction around a center axis of the tube at a position that is an endmost position on a tip side on the reference electrode and faces the heating section,
• an inner connecting section that is provided on an entire periphery or a part in the circumferential direction of the tube at an endmost position on a base end side on the reference electrode and is connected to an inner terminal metal fitting, and
• an inner lead section that is provided on a part in the circumferential direction of the tube at a position where the inner detection section is connected to the inner connecting section and that is formed in a formation region in the circumferential direction smaller than a formation region in the circumferential direction of the inner connecting section;
• the formation region in the circumferential direction of the inner lead section is reduced stepwise or reduced to form a tapered shape from the inner detection section toward the inner connecting section;
• and
• a rate (S1−S2)/S1 has a relationship of 0.3≤(S1−S2)/S1≤0.7, where S1 represents a surface area of an entire portion of the inner surface of the solid electrolyte and S2 represents a surface area of a part of the entire portion of the inner surface of the solid electrolyte on which the reference electrode is formed, the rate (S1−S2)/S1 being a rate of a surface area of a part of the entire portion of the inner surface of the solid electrolyte on which a reduction of the reference electrode is made.