A substrate mounted with an electronic element thereon comprises a metal base, a ceramic insulator, and a radiator. The insulator is provided on a first side of the base. The insulator has a heat-generating electronic element mounted on a side thereof opposite to a side thereof facing the first side of the base. The radiator has a radiation capability higher than both of a radiation capability of the insulator and a radiation capability of the base, and radiates heat transmitted from the electronic element via the insulator to the base.