Abstract: An optical semiconductor device includes: a first conductivity type first semiconductor region; a first conductivity type second semiconductor region formed on the first semiconductor region; a second conductivity type third semiconductor region formed on the second semiconductor region; a photodetector section formed of the second semiconductor region and the third semiconductor region; a micro mirror formed of a trench formed selectively in a region of the first semiconductor region and the second semiconductor region except the photodetector section; and a semiconductor laser element held on the bottom face of the trench. A first conductivity type buried layer of which impurity concentration is higher than those of the first semiconductor region and the second semiconductor region is selectively formed between the first semiconductor region and the second semiconductor region in the photodetector section.