A wiring board includes a first substrate having a penetrating hole penetrating through the first substrate, a built-up layer formed on a surface of the first substrate and including interlayer resin insulation layers and wiring layers, the built-up layer having an opening portion communicated with the penetrating hole of the fist substrate and opened to the outermost surface of the built-up layer, an interposer accommodated in the opening portion of the built-up layer and including a second substrate and a wiring layer formed on the second substrate, the wiring layer of the interposer including conductive circuits for being connected to semiconductor elements, a filler filling the opening portion such that the interposer is held in the opening portion of the built-up layer, and mounting pads formed on the first substrate and positioned to mount the semiconductor elements. The mounting pads are positioned to form a matrix on the first substrate.