Sensor element

A sensor element for a gas sensor measuring concentration of NOx in a measurement gas, said sensor element comprising:

• an oxygen-ion conductive solid electrolyte layer;
• a measurement gas introduction space into which said measurement gas is introduced;
• a reference gas introduction space into which a reference gas is introduced;
• a heater part to heat said sensor element;
• a sensor electrode disposed on one main surface of said solid electrolyte layer to face said measurement gas introduction space, and having an oxygen decomposition ability and a NOx decomposition ability;
• a monitor electrode disposed on said one main surface of said solid electrolyte layer to face said measurement gas introduction space, and having the oxygen decomposition ability;
• and
• a reference electrode disposed on the other main surface of said solid electrolyte layer to face said reference gas introduction space, wherein
• said sensor electrode, said reference electrode, and said solid electrolyte layer constitute a sensor cell as an electrochemical pump cell,
• said monitor electrode, said reference electrode, and said solid electrolyte layer constitute a monitor cell as an electrochemical pump cell, and
• in plan view from a side of said one main surface, a heater element of said heater part overlaps 50% or more of an area of each of said sensor electrode and said monitor electrode.