A wiring board with a built-in electronic component includes a core substrate having a cavity, an electronic component accommodated in the cavity of the core substrate and having a body portion and multiple conductive portions formed on a surface of the body portion, a filling resin filling the space formed in the cavity having the component positioned in the cavity, and an resin insulation layer formed on the core substrate such that the resin insulation layer is covering an opening of the cavity and a surface of the component. The core substrate has an inclination suppressing structure formed on one or more side walls forming the cavity such that the distance between the side wall forming the cavity and the component varies in a portion of the side wall having the inclination suppressing structure and a portion of the side wall other than the portion having the inclination suppressing structure.