Wiring substrate and method for manufacturing wiring substrate

A wiring substrate, comprising:

• an insulating layer comprising resin and filler particles;
• a plurality of conductor layers including an upper-layer conductor layer and a lower-layer conductor layer such that the insulating layer is sandwiched between the upper-layer conductor layer and the lower-layer conductor layer;
• and
• a penetrating conductor formed in the insulating layer such that the penetrating conductor is penetrating through the insulating layer and connecting the upper-layer conductor layer and the lower-layer conductor layer,
• wherein the penetrating conductor is formed such that the penetrating conductor has a first length which is a maximum width of the penetrating conductor in a direction orthogonal to a thickness direction of the wiring substrate and that the first length is 25 inn or less, and the insulating layer is formed such that a maximum particle size of the filler particles in a region within a distance of 40% of the first length from the penetrating conductor is 20% or less of the first length and that the insulating layer does not have the filler particles exposed at an interface between the insulating layer and the penetrating conductor.