The C- and L-bands (1530 nm ∼ 1625 nm) are used for long-haul optical communication systems. The operating wavelength range needs to be extended to realize much broader transmission capacity with a low-cost modulation format. In this paper, we propose a large-scale wavelength multiplexer/demultiplexer using a conventional spatial grating in combination with arrayed-waveguide gratings (AWGs) to cover the whole fiber transmission wavelength range. The diffracted light from the spatial grating is spectrally decomposed and coupled to several AWGs for further demultiplexing. In this study, we designed a multi/demultiplexer which covers the O∼L-bands (186.2 THz ∼ 243.7 THz) with a channel spacing of 100 GHz. This has 9 sub-bands and 576 wavelength channels. The groove density of the spatial grating and the beam diameter on it are assumed to be 300 lines/mm and 1 mm x 1 mm, respectively. The diffracted light is coupled to 9 tapered waveguides on the edge of a planar lightwave circuit (PLC). The shape of the beam is changed by a cylindrical lens, to enhance the coupling efficiency. Each tapered waveguide is connected to an AWG with a center wavelength corresponding to the particular sub-band. AWGs with 64 output channels have a frequency spacing of 100 GHz. The transmission loss in the AWGs was calculated to be between -14.9 dB and -6.6 dB. The relatively large transmission loss was due to misalignment between the beam and the tapered waveguides.