A light-emitting element mounting substrate includes a substrate including insulating resin, a first conductor layer formed on a first surface of the substrate and having an element mounting portion, a second conductor layer formed on a second surface of the substrate on the opposite side of the first surface, metal blocks formed such that the metal blocks are penetrating through the first conductor layer, the substrate and the second conductor layer and positioned in the element mounting portion of the first conductor layer, and through-hole conductors formed adjacent to the metal blocks respectively such that the through-hole conductors electrically connect the first conductor layer and the second conductor layer and that a diameter of each metal block is larger than a diameter of each through-hole conductor.