The object of the invention is to provide a flat high speed differential transmission cable which is composed of a plurality of signal line pairs and can execute low loss without increasing the diameter of the cable. The high speed differential transmission cable is formed in a way that two electric conductors (11) are combined into one group as signal line pair (S(Sp, Sn)) for differential transmission, a plurality of groups of the signal line pairs are arranged in flat shape, and extrude and coat insulating resin to form the cable. The electric conductors (11) composed of a plurality of single core wires or twisted wires are arranged in a column in parallel with an interval P, and are integrally coated by an insulator (12) composed of insulating resin, a tinsel belt (13) is longitudinally attached and coated on the external circumstance of the insulator (12), and an outer skin (15) composed of insulating resin coats the external circumstance of the tinsel belt (13). Ground wires (G) are arranged among the signal wire pairs (S).