An OLT (2) includes one or more optical receivers (22) configured to receive optical signals of respective different wavelengths obtained by an AWG filter (4) demultiplexing a wavelength-multiplexed signal addressed to the terminal itself, and a supervisory controller (23) configured to transmit, to an ONU (3), a wavelength adjustment instruction to transit a wavelength to be used by an optical transmitter (32) for transmission of an optical signal, to set a difference between an optical received power of an optical signal received by any of the optical receivers (22) and a reference value of the optical received power within a threshold, the ONU (3) being a transmission source of the optical signal.