An optical semiconductor, includes a semiconductor substrate having a (100) principal surface, a waveguide mesa stripe formed on a first region of the semiconductor substrate, the waveguide mesa stripe guiding a light therethrough; a plurality of dummy mesa patterns formed on the semiconductor substrate in a second region at a forward side of the first region, and a semi-insulating buried semiconductor layer formed on the semiconductor substrate so as to cover the first and second regions continuously, the semi-insulating buried semiconductor layer filling a right side and a left side of the waveguide mesa stripe in the first region and a gap between the plurality of dummy mesa patterns in the second region.