A substrate for a light-emitting diode comprising a metal base with a thickness of a predetermined value or more is constituted so that the thickness of a top conductor for an electrical connection with a light-emitting diode (LED) in a predetermined range falls within a predetermined range and the thickness of an insulation layer which electrically insulates the metal base and the top conductor and the thickness of the top conductor meet a predetermined relation. Thereby, a substrate for a light-emitting diode which can show a high heat dissipation capacity by achieving a low thermal resistance as the total thermal resistance of the whole substrate without reducing insulation reliability and high-humidity reliability of the substrate is provided.