A sensor including: a sensor element; terminal metal fittings; a terminal surrounding member; and lead wires. The terminal surrounding member is divided into a front side surrounding member and a rear side surrounding member, and a rear end face of the front side surrounding member abuts a front end face of the rear side surrounding member. The sensor includes a fitting structure including a recess portion formed on one of the rear and front end faces and a protruding portion formed on the other, such that one of the front side surrounding member and the rear side surrounding member is prevented from rotating relative to the other about an axis of the sensor, and is prevented from deviating in a direction perpendicular to the axis of the sensor relative to the other.