An arrayed waveguide grating type optical multiplexer/demultiplexer circuit in which wavelength dispersion is reduced. An input wave guide (1), a first slab waveguide (2), an arrayed waveguide (3), a second slab waveguide (4) and an output waveguide (5) are connected sequentially. Furthermore, a parabola waveguide (6) is provided between the input waveguide (1) and the first slab waveguide (2), and a taper waveguide (7) is provided between the second slab waveguide (4) and the output waveguide (5). A parabola waveguide length Z.sub.0 exists in a range Z.sub.a,0=Z.sub.0=Z.sub.p,0 determined by a parabola waveguide length Z.sub.a,0 where the ratio of absolute amplitude between the main peak and the first side peak in the field distribution of far-field of the parabola waveguide (6) has an upper limit of 0.217, and a parabola waveguide length Z.sub.p,0 where the relative phase of the main peak and the first side peak in the field distribution of far-field has a lower limit of 3.14 radian.