An optical network system for bidirectional transmission between a main system (e.g., central station) and a plurality of auxiliary systems (e.g., subscriber's stations). The main system outputs a main optical signal to a main fiber. The auxiliary systems output auxiliary optical signals to a plurality of auxiliary fibers, respectively. The main fiber and the auxiliary fibers are network-connected by a connecting unit. The main system has a detector for detecting the auxiliary optical signals and a control signal generator for generating a control signal including data for designating a specific auxiliary system whose auxiliary optical signal is not detected. The main optical signal in the auxiliary fiber corresponding to the specific auxiliary system is attenuated according to the control signal. Accordingly, laser hazard in each auxiliary system is prevented.