A heat sink has a plurality of radially extending radiating fins, which are arranged annularly on the outer circumferential surface of a cylindrical base to be integral with that surface. The base is formed to have a cylindrical shape having a center axis. The base is formed to be solid-core. The heat sink is ground such that a cooling object contact portion having a generally cylindrical shape projects from the end surface of the heat sink in the axial direction. An annular groove is ground out along the outer circumference of a contact surface of the cooling object contact portion. An attaching member is attached on the outer circumferential surface of the cooling object contact portion, and a pressure application portion is entirely pressed by a pressing machine. The pressure application portion is plastically deformed outward in the diameter direction, so that the attaching member is fixed in between.