An optical directional coupler (1) comprises first and second straight optical waveguides (2, 3) having respective cores (2a, 3a) extending longitudinally close to each other to form an optical coupler portion, and a tapered optical waveguide (4) having a core (4a) connected to the core (2a) of the first straight waveguide (2). The core (2a) of the first straight waveguide (2) has a centerline (2d). A width of the core (4a) of the tapered waveguide (4) becomes narrow toward the first straight waveguide (2) and a profile of the tapered waveguide (4) is asymmetric relative to the centerline (2d) of the core (2a) of the first straight waveguides (2).