A wiring board or an electronic component embedded substrate includes a substrate that includes a resin containing a plurality of fillers; and a via that is electrically connected to at least one interconnect provided to the substrate, wherein the via includes a mix area in which metal is provided between the fillers on an inner radial side with respect to the substrate. A method of manufacturing a wiring board or an electronic component embedded substrate includes preparing a substrate that includes a resin containing a plurality of fillers; forming a via formation hole in the substrate; performing an ashing process on at least an inner wall of the via formation hole; and performing electroless plating on the inner wall of the via formation hole.