A stud pin installable in a tire, comprising: a tip comprising an end surface that comes into contact with a road surface; a body portion holding the tip with the tip projecting from an end portion of one side of the body portion; and a lower flange connected to an end portion of the other side of the body portion opposite to the end portion of the one side of the body portion, wherein a flange contour shape of the lower flange as viewed in an arrangement direction of the tip, the body portion, and the lower flange has an anisotropic shape in which, among imaginary rectangles circumscribing the flange contour shape, at least one smallest rectangle of a first smallest rectangle having the shortest side of four sides being smallest among the rectangles or a second smallest rectangle having the longest side of four sides being smallest among the rectangles comprises short sides and long sides having different lengths, the flange contour shape comprises a first recess portion provided on each of portions of the flange contour shape extending along the short sides, the first recess portion being curved toward a centroid of the flange contour shape, the flange contour shape comprises a second recess portion provided on each of portions of the flange contour shape extending along the long sides, the second recess portion being curved toward the centroid of the flange contour shape, a body contour shape of the body portion as viewed in the arrangement direction is a polygonal shape comprising a plurality of vertex angles, characterized in that the flange contour shape comprises two or more first recess portions provided on each of portions of the flange contour shape extending along the short sides, the number of the vertex angles is identical to the total number of the first recess portions and the second recess portions, and in that the vertex angles are disposed respectively facing the first recess portions and the second recess portions. a tip comprising an end surface that comes into contact with a road surface; a body portion holding the tip with the tip projecting from an end portion of one side of the body portion; and a lower flange connected to an end portion of the other side of the body portion opposite to the end portion of the one side of the body portion, wherein a flange contour shape of the lower flange as viewed in an arrangement direction of the tip, the body portion, and the lower flange has an anisotropic shape in which, among imaginary rectangles circumscribing the flange contour shape, at least one smallest rectangle of a first smallest rectangle having the shortest side of four sides being smallest among the rectangles or a second smallest rectangle having the longest side of four sides being smallest among the rectangles comprises short sides and long sides having different lengths, the flange contour shape comprises a first recess portion provided on each of portions of the flange contour shape extending along the short sides, the first recess portion being curved toward a centroid of the flange contour shape, the flange contour shape comprises a second recess portion provided on each of portions of the flange contour shape extending along the long sides, the second recess portion being curved toward the centroid of the flange contour shape, a body contour shape of the body portion as viewed in the arrangement direction is a polygonal shape comprising a plurality of vertex angles, characterized in that the flange contour shape comprises two or more first recess portions provided on each of portions of the flange contour shape extending along the short sides, the number of the vertex angles is identical to the total number of the first recess portions and the second recess portions, and in that the vertex angles are disposed respectively facing the first recess portions and the second recess portions. a flange contour shape of the lower flange as viewed in an arrangement direction of the tip, the body portion, and the lower flange has an anisotropic shape in which, among imaginary rectangles circumscribing the flange contour shape, at least one smallest rectangle of a first smallest rectangle having the shortest side of four sides being smallest among the rectangles or a second smallest rectangle having the longest side of four sides being smallest among the rectangles comprises short sides and long sides having different lengths, the flange contour shape comprises a first recess portion provided on each of portions of the flange contour shape extending along the short sides, the first recess portion being curved toward a centroid of the flange contour shape, the flange contour shape comprises a second recess portion provided on each of portions of the flange contour shape extending along the long sides, the second recess portion being curved toward the centroid of the flange contour shape, a body contour shape of the body portion as viewed in the arrangement direction is a polygonal shape comprising a plurality of vertex angles, characterized in that the flange contour shape comprises two or more first recess portions provided on each of portions of the flange contour shape extending along the short sides, the number of the vertex angles is identical to the total number of the first recess portions and the second recess portions, and in that the vertex angles are disposed respectively facing the first recess portions and the second recess portions.