A wiring substrate is configured such that a build-up layer is formed only on a front surface (a single side) of a core substrate and such that the distance between a semiconductor device mounted on the front surface thereof and an electronic component mounted on the back surface thereof or incorporated therein behind the back surface is reduced to thereby enhance electrical characteristics of an electrically continuous path therebetween, and whose overall strength is enhanced so as not to be prone to deflection or warpage. The wiring substrate includes a relatively thin first core substrate 2 having a front surface 3 and a back surface 4; a relatively thick second core substrate 6 superposed on the back surface 4 of the first core substrate 2 and having a through opening 9 formed therein, the first substrate 2 and the through opening 9 defining a recess 9; and a build-up layer BU formed on the front surface 3 of the first core substrate 2 and including wiring layers 16 and 25 and dielectric layers 23 and 26.