To provide an optical semiconductor device in which reliability is enhanced by enhancing adhesion strength of a substrate and a sealing resin material, and adhesion strength of the substrate and conductive paste.

An LED 23 is bonded through conductive paste 30 to the mount part 26 of a circuit pattern 25 formed on the surface of a substrate 22 having an insulating basic material 24 and applied with gold plating on the surface, and then it is sealed with an epoxy resin sealing member 33. The mount part 26 has an area not larger than the area at the bonded bottom face of the LED 23.