PURPOSE: To increase the transmission capacity of the whole system in a transparent optical communication system.

CONSTITUTION: An optical communication network is formed by connecting an optical transmission line part constituted of combining an optical fiber 4-2 and an optical amplification part 4-3, plural light transmission parts 4-7, 4-8, 4-9 for transmitting optical multilevel digital signals and plural light receiving parts 4-10, 4-11, 4-12 for receiving an optical multilevel digital signal. When route length between the light transmitting parts 4-7, 4-8 4-9 and the light receiving parts 4-10, 4-11, 4-12 are relatively short (a route 4-6), the number of multilevels of the optical multilevel digital signal is increased, and when the route length is relatively long (a route 4-4), the number of multilevels in the digital signal can be reduced.