Provided is an optical semiconductor device, which includes a GaAs substrate (or a semiconductor substrate) 20; an n-type contact layer (or a doping layer) 21 formed on one surface 20a of the GaAs substrate 20; an active layer 25 formed on top of the n-type contact layer 21 and including at least one quantum dot 23; a p-type contact layer (or a contact layer) 26 formed on top of the active layer 25 and being of an opposite conduction type to the n-type contact layer 21; an insulating layer 29 formed on top of the p-type contact layer 26 and including a first opening 29a whose size is such that a contact region CR of the p-type contact layer 26 lies within the first opening 29a; a p-side electrode layer 33c formed on top of the contact region CR of the p-type contact layer 26 and on top of the insulating layer 29 and including a second opening 33a lying within the first opening 29a; and a n-side electrode layer (or a second electrode layer) 37 formed on the other surface 20b of the GaAs substrate 20.