An object of the present invention is to propose a functional element built-in substrate which enables an electrode terminal of a functional element to be well connected to the back surface on the side opposite to the electrode terminal of the functional element, and which can be miniaturized. According to the present invention, there is provided a functional element built-in substrate including a functional element provided with an electrode terminal on one surface side of the functional element, and a wiring substrate including a laminated structure in which the functional element is embedded so that the electrode terminal of the functional element faces the front surface side of the structure, and which is formed at least in a side surface region of the functional element by laminating a plurality of wiring insulating layers each including a wiring, the functional element built-in substrate being featured in that the electrode terminal and the back surface side of the wiring substrate are electrically connected to each other through the wiring of the laminated structure, and in that, in a pair of the wiring insulating layers included in the laminated structure and that are in contact with each other, the cross-sectional shape of the wiring in each of the wiring insulating layers, which cross-sectional shape is taken along the plane perpendicular to the extension direction of the wiring in the wiring insulating layer, has a relationship that the cross-sectional area of the wiring in the back surface side wiring insulating layer is larger than the cross-sectional area of the wiring in the front surface side wiring insulating layer.