An insulating material high both in thermal conductivity and light reflectance, and a submount high in heat radiatability for mounting an LED element thereon, capable of raising a light utilization factor and quickly radiating heat generated from the element. For example, used as a substrate material of a submount is a nitride sintered body having a reflectance of light in the wavelength region of from 350 nm to 800 nm of 50% or more and a reflectance of light with a wavelength of 700 nm of 60% or more, obtained by sintering a preform consisting of a composition containing 100 parts by mass of aluminum nitride powder and 0.5 to 10 parts by mass of a compound containing an alkaline earth metal such as 3CaO×Al2O3 in an inert atmosphere containing a specific quantity of carbon vapor, or by burning a coat of a nitride paste applied on a base substrate having a heat resistance at a predetermined temperature.