A wiring board for a built-in electronic component includes a first insulating layer, a second insulating layer formed under the first insulating layer, and a conductor layer formed on an upper surface of the second insulating layer such that a cavity is formed to penetrate through the first insulating layer and the conductor layer and expose the second insulating layer at a bottom of the cavity and is formed to accommodate an electronic component therein. The first insulating layer and the conductor layer are formed such that the cavity has a first inner side surface extending from an upper opening edge to a position closer to the second insulating layer, and a lateral expansion part formed between a lower edge of the first inner side surface and the second insulating layer and extending outward from the lower edge of the first inner side surface.