To provide optical transmission line and system of a wide band and small absolute value of cumulative wavelength dispersion with a low loss and easily dealt with.

The optical transmission system 1 is provided with a relay 31, a transmitting optical fiber 41 and the relay 32 as the optical transmission line 2 at least in a part of a transmission path of a signal beam from a transmitter 10 to a receiver 20. The transmitting optical fiber 41 whose wavelength dispersion is +4-+10 ps/nm/km in a wavelength 1.55 μm, and whose dispersion slope is 0-0.04 ps/nm2/km, is arranged in a relay section from the relay 31 to the relay 32. A dispersion compensative optical fiber 313 whose wavelength dispersion is -40 ps/nm/km or below at a wavelength 1.55 μm, and whose dispersion slope is -0.10 ps/nm2/km or below, is wound in a coil shape, and is made a module to be provided in the relay 31.