An electronic element mounting substrate includes: a first substrate including a first principal face; a second substrate located inside the first substrate in a plan view of the electronic element mounting substrate, the second substrate being made of a carbon material; a third substrate located between the first substrate and the second substrate in the plan view, the third substrate being made of a carbon material; and a first mounting portion for mounting a first electronic element, the first mounting portion being located on the first principal face side in a thickness direction of the substrate. The second substrate and the third substrate each have a low heat conduction direction and a high heat conduction direction. The second substrate and the third substrate is arranged so that the low heat conduction directions thereof are perpendicular to each other, and the high heat conduction directions thereof are perpendicular to each other.