a flat cable includes a signal line extending in a longitudinal direction , a thin dielectric sheet with which the signal line is coated and that has plasticity , a pair of spaced apart ground layers extending in the longitudinal direction and sandwiching the dielectric sheet in its thickness direction , and insulators that coat the pair of ground layers so that they are not exposed to the outside . the cross - sectional size of the signal line in a direction orthogonal to the longitudinal direction , the thickness and width of the dielectric sheet , and so forth are selected to obtain a predetermined characteristic impedance for the cable . each of the pair of ground layers is sized so as to be substantially wider than the signal line .