A diffraction grating of the present invention includes a first block (101) and a second block (102). The first block (101) is made up of a plurality of input waveguides (103), a slab waveguide (104), a delay waveguide array (105) and narrow grooves (106): each of which being filled with resin. The second block (102) is made up of a plurality of input waveguides (107), a slab waveguide (108) and a plurality of output waveguides (109). Since a circuit is cut off at the ends of the arrayed waveguides, a relative phase can easily be measured. The resin has undergone a refractive-index adjustment and compensates a phase error.