In a stud pin, a flange contour shape of a lower flange is an anisotropic shape in which, among imaginary rectangles circumscribing the contour shape, the smallest rectangle includes short sides and long sides having different lengths. The flange contour shape includes first recess portions on each of portions of the flange contour shape extending along the short sides, the first recess portions being curved toward a centroid of the flange contour shape, and the flange contour shape includes a second recess portion on portions of the flange contour shape extending along the long sides, the second recess portions being curved toward the centroid of the flange contour shape. A body contour shape of a body portion is a polygonal shape including vertex angles in a same number as the first and second recess portions, and the vertex angles are disposed facing the first and second recess portions.