A temperature sensor includes a metal tube containing thermo-sensitive element and filled with an insulating filling material, and a housing to which the metal tube is fixed. The metal tube includes a small-diameter part and a large-diameter part. The distal end of the small-diameter part is located beyond a virtual line. The virtual line passes a center of a maximum virtual circle which is an inscribed circle that contacts at least three points on an inner surface of the pipe in a cross-section perpendicular to an axial direction of the pipe, and is perpendicular to a center axis line of the metal tube. The whole of the small-diameter part and the part of the large-diameter part filled with the filling material are located within a center virtual circle which is coaxial with the maximum virtual circle and has an inner diameter of ⅔ of that of the maximum virtual circle.