In an optical wavelength division multiplexed access system, a center unit (OSU) and n optical network units (ONUs) are connected together via a wavelength splitter and optical fiber transmission lines, and downstream optical signals from the OSU to the ONUs and upstream optical signals from the ONUs to the OSU are transmitted in both directions, the wavelength spacing .DELTA..lamda.d (optical frequency spacing .DELTA.fd) of the downstream optical signals is set to twice or more the wavelength spacing .DELTA..lamda.u (optical frequency spacing .DELTA.fu) of the upstream optical signals, each ONU transmits an upstream optical signal whose optical spectral width is twice or more .DELTA..lamda.u (.DELTA.fu), and the wavelength splitter spectrum slices the upstream signals transmitted from the ONUs into wavelengths (optical frequencies) whose optical spectral widths are mutually different within .DELTA..lamda.u (.DELTA.fu), and wavelength-division multiplexes them and transmits to the OSU.