In an electronic component built-in wiring substrate, an electronic component is mounted on a first wiring substrate. A second wiring substrate is stacked on the first wiring substrate and is connected electrically to the first wiring substrate by connection terminals. The second wiring substrate has an opening portion of a size larger than a planar area of the electronic component. An underfill resin is filled in a first space between the first wiring substrate and the electronic component, and has a raised portion which is raised along an outer peripheral side surface of the electronic component, seals a clearance between an inner peripheral edge of the opening portion and an outer peripheral edge of the electronic component and supports the second wiring substrate. A sealing resin is filled in a second space between the first and second wiring substrates.