PURPOSE: To detect a collision irrespective of contents of transmitting data, a modulating method, a transmission rate, etc. by superposing an optical bias on an optical signal transmitted from each terminal, and detecting a collision of a signal when a star repeater receives two or more optical biases at the same time.

CONSTITUTION: An optical bias (a) is superposed on an optical signal transmitted from each terminal 1i so that a collision is detected when a collision detecting part 7 of an optical star repeater 2 receives two or more optical biases at the same time. In case when the collision is detected, relaying is halted by a gate 8, and also a collision signal is sent out to a terminal. In this way, even if a transmitting signal is logical L, the collision can be detected exactly. Also, this device is used for detecting the bias (a) superposed on the signal, therefore, the collision is detected irrespective of a transmission rate of the signal, a frequency characteristic, etc.