To provide a wavelength multiplex PON system capable of inexpensively constituting an optical branch station in a PON system in which a plurality of slave stations are connected to a master station by optical fibers via the optical branch station.

The master station is connected to an optical branch station 3 by a main optical fiber 2, and the optical branch station 3 is connected to the plurality of slave stations 3 by branch optical fibers 4. The optical branch station 3 is provided with an optical coupler 34 for multiplexing incoming optical signals from each slave station to the master station which have passed through the branch optical fiber 4; an optical path dividing device 31 for sending the optical signal multiplexed by the optical coupler 34 out to the master station through the main optical fiber 2, and supplying outgoing optical signals to the slave stations which are received from the master station through the main optical fiber 2; and an optical demultiplexer 33 for multiplexing an outgoing optical signal supplied from the dividing device 31, according to the wavelength included in the optical signal and sending out it to each of the slave stations through each of the branch optical fibers 4.