A substrate for mounting a light-emitting element according to the present disclosure contains a crystal particle of aluminum oxide and is composed of an alumina-based ceramic that contains 97% by mass or more of Al as a value of an Al2O3 equivalent among 100% by mass of all components thereof. An average value of an equivalent circle diameter of the crystal particle is 1.1 μm or greater and 1.8 μm or less and a standard deviation of an equivalent circle diameter thereof is 0.6 μm or greater and 1.4 μm or less.