To provide a wiring substrate having excellent transmission characteristics, of which initial failure of a plating layer formed on an inner wall surface of a hole is suppressed regardless of the type of the pre-treatment applied to the inner wall surface of the hole, and of which the plating layer has favorable heat resistance, and a process for producing it.A wiring substrate 10 comprising an electrical insulator layer 20, a first conductor layer 32 formed on a first surface of the electrical insulator layer 20, a second conductor layer 34 formed on a second surface of the electrical insulator layer 20, and a plating layer 42 provided on an inner wall surface of a hole 40 which opens from the first conductor layer 32 through the second conductor layer 34; wherein the electrical insulator layer 20 has a heat resistant resin layer 22 containing a heat resistant resin and a resin powder; the resin powder is formed from a resin material containing a melt-formable fluororesin having a functional group such as a carbonyl group-containing group; the content of the resin powder is from 5 to 70 mass % to the heat resistant resin layer 22; and the electrical insulator layer 20 has a dielectric constant of from 2.0 to 3.5.