A stud pin includes a tip and a body portion provided around a center axis of the stud pin. The body portion includes an upper flange, a lower flange, and a shank portion connecting the upper flange and the lower flange. A first side surface of the lower flange includes a first curved surface recessed inward in relation to the body portion. A second side surface of the upper flange includes a second curved surface bulging toward a periphery of the body portion, and a pair of flat surfaces configured to connect the second curved surface to interpose the second curved surface. A trough portion where the first curved surface is most recessed and a peak portion where the second curved surface is most bulged are located at an identical orientation position about the center axis as viewed from the axial direction.