To provide a technique that enables reduction of crack generation in a connection part.

A sensor element includes a plate-shaped solid electrolyte layer extending in the lengthwise direction; an electrode part laminated on the solid electrolyte layer; and a lead part laminated on the solid electrolyte layer, which extends in the lengthwise direction while electrically connected to the electrode part and is narrower in width than the electrode part. The sensor element has a connection part which is positioned between the electrode part and the lead part and connects the electrode part to the lead part. The width of the lead part is less than 50% of that of the electrode part. Both side faces of the connection part have a round surface of which radius is 0.4 mm or more in inward protrusion when the sensor element is viewed in the lamination direction.