Abstract: A signal transmission flat cable is provided with an upper electrically insulating thin-film layer (4) and a lower electrically insulating thin-film layer (5) that cover a signal conductor (1), ground conductors (2, 2?), and an electrically insulating substrate (3) from above and below in the cable-thickness direction. A protective shielding layer (8) comprised of a metallic layer (6) and an electrically insulating plastic layer (7) surrounds the upper and lower electrically insulating thin-film layers so that the metallic layer is located inside and the electrically insulating plastic layer is located outside. The protective shielding layer surrounds the outer periphery of the upper and lower electrically insulating thin-film layers such that one end edge (10) and the other end edge (11) abut against each other in the cable-length direction. An abutting part (12) formed by abutting the two end edges of the protective shielding layer is located above the ground conductor (2).