A wavelength-division multiple access system having a center apparatus, n optical network units (ONU 1 to ONU n), and m optical networks units (ONU n+1 to ONU n+m) arranged via a wavelength multi/demultiplex apparatus, the center apparatus and the wavelength multi/demultiplex apparatus being connected together in a multiplex section (21a, 21b) via a multiplex section optical fiber, the wavelength multi/demultiplex apparatus and each of the optical network units (ONU 1 to ONU n+m) being connected together in an access section via access section optical fibers, wherein the multiplex section (21a, 21b) is configured to transmit downlink optical signals from the center apparatus to the optical network units (ONU 1 to ONU n+m) and uplink optical signals from the optical network units (ONU 1 to ONU n+m) to the center apparatus using wavelengths assigned to the respective optical network units (ONU 1 to ONU n+m) while multiplexing the wavelengths, the wavelength multi/demultiplex apparatus is configured to carry out wavelength multiplexing or demultiplexing for bidirectional transmissions, wherein a wavelength band Da of wavelengths λd1 to λdn for downlink optical signals corresponding to the n optical network units (ONU 1 to ONU n), a wavelength band Ua of wavelengths λu1 to λun for uplink optical signals corresponding to the n optical network units (ONU 1 to ONU n), a wavelength band Db of wavelengths λdn+1 to λdn+m for downlink optical signals corresponding to the m optical network units (ONU n+1 to ONU n+m), and a wavelength band Ub of wavelengths λun+1 to λun+m for uplink optical signals corresponding to the m optical network units (ONU n+1 to ONU n+m) are set different from one another, the wavelength bands Ua and Ub are set adjacent to each other, and the wavelength bands Ua and Da and/or the wavelength bands Ub and Db are set adjacent to each other, said center apparatus comprises: means for multiplexing said downlink optical signals multiplexed in the wavelength band Da of wavelengths λd1 to λdn and said downlink optical signals multiplexed in the wavelength band Db (wavelengths λdn+1 to λdn+m) and then transmitting to the multiplex section optical fibers, means for demultiplexing the uplink optical signals transmitted via the multiplex section optical fibers into said uplink optical signals multiplexed in the wavelength band Ua of wavelengths λu1 to λun and said uplink optical signals multiplexed in the wavelength band Ub of wavelengths λun+1 to λun+m, said wavelength multi/demultiplex apparatus comprises: means for demultiplexing the downlink optical signals transmitted via the multiplex section optical fibers into said downlink optical signals multiplexed in the wavelength band Da of wavelengths λd1 to λdn and said downlink optical signals multiplexed in the wavelength band Db of wavelengths λdn+1 to λdn+m, means for multiplexing said uplink optical signals multiplexed in wavelength band Ua of wavelengths λu1 to λun and said uplink optical signals multiplexed in wavelength band Ub of wavelengths λun+1 to λun+m and then transmitting to the multiplex section optical fibers, and each of the optical network units (ONU 1 to ONU n+m) comprises: downlink optical signal receiving means for receiving a downlink optical signal of one of the wavelengths λd1 to λdn+m in the wavelength bands Da and Db which wavelength is assigned to the respective optical network unit (ONU 1 to ONU n+m); and uplink optical signal transmitting means for transmitting an uplink optical signal of one of the wavelengths λu1 to λun+m in the wavelength bands Ua and Ub which wavelength is assigned to the respective optical network unit (ONU 1 to ONU n+m) or an uplink optical signal within a broad band including the wavelength bands Ua and Ub. means for multiplexing said downlink optical signals multiplexed in the wavelength band Da of wavelengths λd1 to λdn and said downlink optical signals multiplexed in the wavelength band Db (wavelengths λdn+1 to λdn+m) and then transmitting to the multiplex section optical fibers, means for demultiplexing the uplink optical signals transmitted via the multiplex section optical fibers into said uplink optical signals multiplexed in the wavelength band Ua of wavelengths λu1 to λun and said uplink optical signals multiplexed in the wavelength band Ub of wavelengths λun+1 to λun+m, means for demultiplexing the downlink optical signals transmitted via the multiplex section optical fibers into said downlink optical signals multiplexed in the wavelength band Da of wavelengths λd1 to λdn and said downlink optical signals multiplexed in the wavelength band Db of wavelengths λdn+1 to λdn+m, means for multiplexing said uplink optical signals multiplexed in wavelength band Ua of wavelengths λu1 to λun and said uplink optical signals multiplexed in wavelength band Ub of wavelengths λun+1 to λun+m and then transmitting to the multiplex section optical fibers, and downlink optical signal receiving means for receiving a downlink optical signal of one of the wavelengths λd1 to λdn+m in the wavelength bands Da and Db which wavelength is assigned to the respective optical network unit (ONU 1 to ONU n+m); and uplink optical signal transmitting means for transmitting an uplink optical signal of one of the wavelengths λu1 to λun+m in the wavelength bands Ua and Ub which wavelength is assigned to the respective optical network unit (ONU 1 to ONU n+m) or an uplink optical signal within a broad band including the wavelength bands Ua and Ub.