To provide a differential signal transmission circuit formed on a flexible printed board capable of controlling the width and intervals of signal wire with high accuracy, making it possible to obtain desired differential impedance characteristic and noise resistance.

A differential signal transmission circuit comprises a base film 1 of a single-sided flexible printed board 100, a GND pattern composed of ground (GND) wires 3 formed on one side of the base film 1, and a signal transmission pattern composed of a pair of signal wires 4 formed between the GND wires 3. The differential signal transmission circuit is formed in such a way that the distance D between the GND wires 3 and the signal wires 4 and the distance S between the signal wires 4 are respectively the same (i.e., distance D = distance S), the distance being set to a value from 30 μm to -1/20(3600-150Zdiff+Zdiff²) μm, both ends inclusive, or to a value equal to or less than 1/20(12000-210Zdiff+Zdiff²) μm.