PURPOSE: To detect a collision at a low cost and with high reliability, by monitoring a receiving optical level by a collision detecting device installed adjacently to a star coupler.

CONSTITUTION: When a transmitting signal is encoded by an encoder 16, and is sent out from a transmitter 2, the signal is sent to all stations connected through a star coupler 4, and also is inputted to a collision detecting device 19 connected to a pair of short ports of the coupler 4. Subsequently, when optical signals from two or more stations are sent out simultaneously into the coupler, and a collision of the optical signals occurs, an output of an ampilfier 21 of a device 19 increases, an output of an integration circuit 22 exceeds a threshold level of a terminal 24 of a discriminator 23, a driving signal is outputted to a light emitting element 26 from a transmitter 25, and a collision alarm signal of a DC optical output is sent out. In the station which sends out the optical signal, a DC signal of an amplifier 8 increases, an output of the circuit 10 exceeds a threshold level 14, and a collision with a signal of other station is detected.