An object of the present invention is to provide a germanium laser diode that can be easily formed on a substrate such as silicon by using a normal silicon process and can emit light efficiently. A germanium light-emitting device according to the present invention is a germanium laser diode characterized in that tensile strain is applied to single-crystal germanium serving as a light-emitting layer to be of a direct transition type, a thin semiconductor layer made of silicon, germanium or silicon-germanium is connected adjacently to both ends of the germanium light-emitting layer, the thin semiconductor layer has a certain degree of thickness capable of preventing the occurrence of quantum confinement effect, another end of the thin semiconductor layer is connected to a thick electrode doped with impurities at a high concentration, the electrode is doped to a p type and an n type, a waveguide is formed so as not to be in direct contact with the electrode, and a mirror is formed at an end of the waveguide.