An electronic component built-in substrate comprises a substrate having a core member with an opening in which an electronic component is disposed, a first auxiliary insulating layer formed on a first surface of the core member; a second auxiliary insulating layer formed on a second surface of the core member, the second auxiliary insulating layer having a first via hole, a filling resin portion filling a gap between the electronic component and a side surface of the opening of the core member, and a first wiring layer formed on the second auxiliary insulating layer and connected to the connection terminal of the electronic component through the first via hole. The whole of the first surface and the whole of the second surface of the core member are in direct contact with the first auxiliary insulating layer and the second auxiliary insulating layer, respectively.