A first substrate includes a first surface and a second surface opposite to the first surface. A second substrate includes a third surface and a fourth surface opposite to the third surface. A third substrate includes a fifth surface and a sixth surface opposite to the fifth surface. The first substrate is made of an insulator, and includes a mounting portion for mounting an electronic element at the first surface, and the mounting portion for mounting the electronic element is a rectangular shape. The third substrate is made of a carbon material, and the fifth surface is connected to at least the second surface at location overlapped with the mounting portion for mounting the electronic element in plan view. The third substrate has a larger heat conduction in a direction perpendicular to the longitudinal direction of the mounting portion than heat conduction in the longitudinal direction of the mounting portion in plan view.