Abstract: To provide a substrate for mounting a light-emitting element having a reflectance increased since light which is transmitted through the substrate and leaks out in directions other than the direction from which the light enters is reduced, and having a reduced firing shrinkage in the plane direction. A substrate for mounting a light-emitting element of the present invention is made of a glass ceramic 10 containing glass 1 and a flat oxide type ceramic filler 2 having an average aspect ratio of at least 25 dispersed in the glass 1. The ceramic filler 2 is contained in a state where it is aligned in parallel with the plane direction of the substrate, and the glass ceramic 10 contains no crystals except for the ceramic constituting the filler. This substrate has an extremely high reflectance of at least 84% (thickness: 300 ?m) to light in the visible region.