This invention discloses an optical transmission device used for bi-directional optical communications. The optical transmission device comprises a uni-directional optical signal processing unit for performing specified optical signal processing for optical signals transmitted in a single direction and a uni-direction/bi-direction changing unit for unifying the flows of clockwise and counterclockwise optical signals in a single direction, inputting these flow-unified optical signals to the uni-directional optical signal processing unit and dividing the flow of optical signals from the uni-directional optical signal processing unit between clockwise and counterclockwise directions. Bi-directional wavelength-division multiplexing optical communications can be performed by unifying the transmission routes (flows) of optical signals transmitted in two ways in a single direction and using the existing optical transmission device for uni-directional optical communications.