An optical element-mounting substrate that makes it easy to optically couple the optical semiconductor element to the optical fiber or to the lens and that can be highly integrated while suppressing deterioration in the high-frequency signals. The optical element-mounting substrate for optically coupling the optical semiconductor element to the optical fiber through the lens, comprises an insulating film formed on the surface of the optical element-mounting substrate, grooves formed on the substrate for installing the optical fiber and the lens, a thin-film electrode formed on the insulating film, a thin-film capacitor and a thin-film temperature sensor arranged maintaining a distance from the thin-film electrode, and a solder film formed on the insulating film in a region where the optical semiconductor element is mounted so as to be electrically connected to the thin-film electrode.