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.