For a WDM optical communication, on a substrate of an arrayed waveguide type wavelength multiplexing/demultiplexing device, input waveguide propagates a plurality of light and outputs WDM light; a first slab diffuses the WDM light outputted from the input waveguide; a channel waveguide array separates the WDM light and causes the separated six lights to propagate; a second slab condenses the separated six lights having propagated through the channel waveguide array; output waveguides causes the separated six lights condensed to propagate, and a waveguide interval in first neighborhoods connected to the first slab is set to become wider than a first waveguide interval in a first connection section for connecting the channel waveguide array to the first slab, which achieves a reduced chip size, cost-reduction of chips, and reduces loss.