The present invention is an optical semiconductor device including a lower clad layer 12 having a first conduction type, an active layer 14 that is provided on the lower clad layer 12 and has multiple quantum dot layers 51-55 having multiple quantum dots 41, and an upper clad layer 18 that is provided on the active layer 14 and has a second conduction type opposite to the first conduction type, the multiple quantum dot layers 51-55 having different quantum dot densities.