A side surface type optical semiconductor device includes a substrate made of an insulating material and having a main surface and a back surface, which face opposite sides to each other in a thickness direction. The substrate includes a first concave portion recessed in the thickness direction and a second concave portion recessed further toward the back surface than the first concave portion, a semiconductor optical element is disposed across the first concave portion and the second concave portion, a hollow portion is formed between the semiconductor optical element and the second concave portion, and the hollow portion is used as a light guide path of the semiconductor optical element.