A component built-in board mounting body has a component built-in board mounted on a mounting board, the component built-in board being configured having stacked therein a plurality of printed wiring bases that each have a wiring pattern and a via formed on/in a resin base thereof, and having an electronic component built in thereto, wherein the component built-in board has at least a portion of the plurality of printed wiring bases including thermal wiring in the wiring pattern and including a thermal via in the via, and is mounted on the mounting board via a bump formed on a surface layer of the component built-in board, and a surface on an opposite side to an electrode formation surface of the built in electronic component is connected to the bump via the thermal via and the thermal wiring, and is thermally connected to the mounting board via the bump.