The present invention provides a heat spreader 1 which includes a substrate 7 composed of a metal-containing material and in which a second-component connection surface 6 of the substrate 7 is provided with wettability with a solder and a solder block layer 14 is formed in at least one of respective regions, adjacent to each other, of the second-component connection surface 6 and a side surface 13. When a semiconductor device 4 is manufactured by connecting a semiconductor laser 2 to the element connection surface 5 through an element connecting layer 8, and then connecting a heat sink 3 to the second-component connection surface 6 through a second-component connecting layer 9 having a melting point lower than that of the element connecting layer 8, the semiconductor block layer 14 can inhibit the melted solder from creeping up along the side surface 13, which can prevent defects that the solder that has crept up short-circuits the semiconductor laser 2 and blocks a luminous flux of laser light emitted from an emission surface 12 of the semiconductor laser 2.