An optical transmission system with automatic startup functions to optimize the power level of optical signals entering each optical amplifier and operate those amplifiers in appropriate mode. In the process of starting up an optical transmission system, upstream and downstream optical transmission devices set up themselves under the coordination of their internal controllers. The downstream device controller adjusts its local variable optical attenuator by utilizing optical noise emission from a post-amplifier in the upstream optical transmission device, and it selects and sets a preamplifier unit to work in appropriate operating mode. The upstream device controller, on the other hand, selects and sets operating mode of its local post-amplifier unit. During this process, the upstream and downstream device controllers exchange messages over supervisory control channels to achieve the purpose.