A method of manufacturing an optical semiconductor element includes: a first step in which a columnar structure of a semiconductor layer formed on a semi-insulating substrate is formed; a second step in which the substrate is exposed in a periphery of the columnar structure; a third step in which a region including exposed surfaces of the first contact layer and the substrate is pretreated; a fourth step in which a first electrode is formed on the exposed surface of the first contact layer; a fifth step in which an interlayer insulating film is formed in a region including a side surface of the columnar structure and the exposed surfaces; a sixth step in which a first electrode wiring is formed on the interlayer insulating film; and a seventh step in which a second electrode wiring is formed on the interlayer insulating film.