A transmission cable that has three signal lines, and in which there is little unwanted radiation noise, is provided. In a section that is perpendicular to the longitudinal direction of the differential transmission cable, the distance between any two signal lines of the three signal lines is equal to a predetermined value. The differential transmission cable is twisted in its longitudinal direction. The differential transmission cable further includes a dielectric core line, and the signal lines are formed on the surface of the dielectric core line. In the differential transmission cable, the signal lines are formed in a helix in the longitudinal direction of the dielectric core line.