An optical semiconductor device includes: a beam splitter that splits an input optical axis into a first split axis having a first split angle and a second split axis having a second split angle larger than the first split angle; a first unit that is located on the first split axis of the beam splitter and has one or more optical components, an interval between a more distant end of the first unit and the beam splitter having a first length; a second unit that is located on the second split axis of the beam splitter and has one or more optical components, an interval between a more distant end of the second unit and the beam splitter having a second length larger than the first length; and an optical semiconductor element that has a first outputting end having a first output axis coupled optically to the input optical axis of the beam splitter, a second outputting end having a second output axis, and optical gain, the optical semiconductor element being inclined so that the second output axis is arranged away to a side of the second split axis from the first output axis.