An electronic element mounting substrate includes a first substrate that has a first main surface, has a rectangular shape, and has a mounting portion for an electronic element on the first main surface, and a second substrate that is located on a second main surface opposite to the first main surface, is made of a carbon material, has a rectangular shape, has a third main surface facing the second main surface and a fourth main surface opposite to the third main surface, in which the third main surface or the fourth main surface has heat conduction in a longitudinal direction greater than heat conduction in a direction perpendicular to the longitudinal direction, and that has a recessed portion on the fourth main surface.