The invention provides an optical transmission system which can perform bidirectional optical transmission at a low cost and with a low power consumption while reducing the space required for optical cables. The optical transmission system includes: a core portion (33a) for transmitting a clock signal as an optical signal; a core portion (33b) for transmitting a data signal or a control signal as an optical signal in a first direction identical to the transmission direction of the clock signal; and a core portion (33c) for transmitting the data signal or the control signal as the optical signal in a second direction opposing to the transmission direction of the clock signal. The data signal or the control signal transmitted in the first direction is synchronized with the clock signal and transmitted. The data signal or the control signal transmitted in the second direction is synchronized with the clock signal transferred via the core portion (33a). The data signal or the control signal transmitted via the core portion (33c) is synchronized with the clock signal transmitted in the first direction. Thus, the data signal or the control signal can be transmitted in both directions.