Wiring substrate and component built-in wiring substrate

A wiring substrate, comprising:

• a core substrate;
• and
• a build-up part laminated on a surface of the core substrate and comprising a plurality of resin insulating layers,
• wherein the plurality of resin insulating layers includes a first resin insulating layer, the build-up part includes a conductor layer formed on the first resin insulating layer, a second resin insulating layer laminated on the first resin insulating layer and covering the conductor layer, and a recess part penetrating through the second resin insulating layer and exposing a portion of the conductor layer such that the conductor layer includes a component mounting region configured to place an electronic component in the recess part and a conductor pad forming a bottom surface of the recess part, the plurality of resin insulating layers includes inorganic filler such that all of the resin insulating layers or all of the resin insulating layers other than the first resin insulating layer include the inorganic filler and that an inorganic filler content rate of the first resin insulating layer is lower than an inorganic filler content rate of the resin insulating layers other than the first resin insulating layer.