To transmit a high-speed digital signal of several tens GHz via a differential line by connecting a differential line referring to the ground to differential lines not referring to the ground, there is provided a signal transmission system which transmits a digital signal between circuit blocks via a signal transmission line, each of the circuit blocks basically including a functional circuit, a reception/transmission circuit formed separately from the functional circuit and an impedance-matched transmission line (115) formed between reception and transmission ends of the reception/transmission circuit; a differential line (105) referring to the ground (110), led out from a differential output driver, being formed from differential signal lines disposed symmetrically with respect to the ground (110) in the circuit block, only differential pair lines (111, 112) not referring to the ground being extended directly from the differential signal lines disposed symmetrically with respect to the ground in the signal transmission line (115).