An optical semiconductor device includes: a waveguide unit which is formed on a semiconductor substrate including a (100) plane and includes a core layer which propagates light; a spot size converting unit which is formed on the semiconductor substrate, is optically connected to the waveguide unit, and converts diameter of light propagated; and a pair of terraces which are formed on the semiconductor substrate and are opposed to each other while sandwiching the spot size converting unit. Interval between opposed units which are opposed to each other while sandwiching the spot size converting unit in the pair of terraces changes, and each of the opposed units includes a part whose orientation tilts to a [0-11] direction with respect to a [011] direction, and position of an upper end of the spot size converting unit is higher than that of an upper end of the waveguide unit.