A differential signal transmission cable is composed of a Twinax cable including twin electrically insulated wires, which are arranged side by side in contact with each other, and a drain wire, which is arranged in contact with and parallel to both of the twin electrically insulated wires, and a shield tape, which is wound around a circumference of the Twinax cable including the drain wire. When in cross sectional view, an isosceles triangle is defined as having, as its base, a line segment that joins respective centers of the twin electrically insulated wires, and as its vertex point, a center of the drain wire, the isosceles triangle has a vertex angle of not smaller than 74 degrees and not greater than 90 degrees.