An arrayed waveguide grating with a first slab waveguide connected to optical input waveguides and to arrayed waveguides for propagating light transmitted from the first slab waveguide. A second slab waveguide is connected to an output end of the arrayed waveguides and to optical output waveguides. The fluctuation in the refractive index of a core in its propagation direction over entire lengths of the arrayed waveguides is restrained. The standard deviation of a phase error distribution generated over the entire lengths of the arrayed waveguides in the propagation direction is restrained to 0.6 rad or less.