An optical waveguide provided with a slab waveguide, which has a plurality of phase gratings arranged at a distance from each other in a direction substantially parallel to a light propagation direction and diffracting propagated light and a plurality of interference regions arranged alternately to the plurality of phase gratings in the direction substantially parallel to the light propagation direction and interfering the light diffracted by the plurality of phase gratings, and an arrayed waveguide whose end is connected to an end of the slab waveguide at a position of a constructive interference portion of a self-image formed by the plurality of phase gratings as an integrated phase grating.