To provide an optical transmission system capable of omitting the transmission of additional information for training at the time of operation to transmit data from a slave station to a master station.

During a training term T1 to be run before an operating term T2 for transmitting data to be originally transmitted, training data concerning respective slave stations 611-613 are sent to the master station. The master station measures the generation timing of reception levels or reception data for each of slave stations 611-613 while using these training data and stores the result in a storage part. During the following operating term T2, concerning optical signals 611-613 sent from the slave stations 611-613, the generation timing of reception levels or reception data thereof is adjusted by a value stored in the storage part. Since data for adjustment are stored during the training term T1, the transmission of the additional information for training during the operating term T2 can be omitted.