To provide an optical waveguide, wherein problems due to positional deviation in connecting the optical waveguide are solved.

The multimode optical waveguide (3-1) has a grating (3-5) in a tapered part (3-4). If the wave vector of the grating formed in a direction crossing the guiding direction of light, when positional deviation is caused during the connection, a low-order mode light is excited from a high-order mode so as to stabilize inter-mode energy. Consequently, connection loss of the multimode optical waveguide is suppressed low, thereby suppressing fluctuation of an excitation mode due to the positional deviation during the connection. As a result, while insertion loss is kept constant, stability of a branching ratio of a branch circuit and an extinction ratio of a WDM circuit is secured.