An optical waveguide includes a first waveguide region and a second waveguide region. The first waveguide region includes a first core, a first clad provided around the first core, and a light reflecting surface. The light reflecting surface has at least one function of (i) reflecting light from a longitudinal direction of the first core toward a direction crossing the longitudinal direction and (ii) reflecting light from the crossing direction toward the longitudinal direction of the first core. The second waveguide region continues from the first waveguide region. The second waveguide region that includes a second core, and a second clad provided around the second core. The second core of the second waveguide region is thinner than the first core of the first waveguide region. A total thickness of the second core and the second clad is smaller than that of the first core and the first clad.