PURPOSE: To make a submount small in variation of resistance in a thicknesswise direction after an assembly operation is finished and to make a semiconductor laser stable in characteristics by a method wherein a first Au layer, an AuSb layer, and a second Au layer are successively metallized on both the sides of a sub-mount base doped with Sb.

CONSTITUTION: A first Au layer 11, an AuSb layer 12, and a second Au layer 13 are successively metallized on both the sides of a sub-mount base 2 doped with Sb. Therefore, when a semiconductor laser chip 6 is fixed onto a metal block 7, Sb contained in the AuSb layer 12 between the first Au layer 11 and the second Au layer 13 is diffused into an AuSi eutectic crystal solder with Sb contained in the sub-mount base 2 and mixed together to form a uniform AuSiSb eutectic crystal solder layer. By this setup, a sub-mount becomes very small in variation of resistance in a thicknesswise direction and a semiconductor laser chip stable in characteristics can be realized.