Abstract: In an optical network system comprising a plurality of communication stations (21) and a star coupler (22) common to the stations, a collision test signal is produced from a generator (50) included in a transmitter section (31) of each station to be sent to the star coupler prior to transmission of a data sequence and is directly returned from the star coupler to a receiver section (32) thereof. The receiver section comprises a detector (51) for monitoring the collision test signal to detect occurrence of collision. The detector makes the transmitter section intercept the data sequence on occurrence of the collision. Preferably, the collision test signal is encoded by an encoder (37) in the transmitter section to be combined with a snychronizing signal and is decoded by a decoder (42) in the receiver section.