Transmission line structure is composed of a pair of signal conductors which are embedded in one wiring region of a dielectric layer and a thickness in height of the signal conductor is larger than a width, and is constituted so that a coupling impedance between the adjacent signal conductors is lower than a coupling impedance between the signal conductor and another conductor formed in another wiring region, and thus to provide a multi-layer wiring board having a transmission line structure of high wiring density and excellent transmission characteristic.