A transmitting-end device generates a pilot signal with a pilot signal generation section, and transmits a pilot signal to a receiving-end device. At the receiving-end device, a transmission rate modification section detects the transmission band of an optical transmission line based on the amplitude of the pilot signal, and decides a data transmission rate acceptable to the receiving-end device by taking into account the transmission band of the optical transmission line. Based on a maximum data transmission data acceptable to the transmitting-end device and the data transmission rate thus decided, a control section in the receiving-end device arbitrates a data transmission rate between it and the transmitting-end device.