According to one embodiment, an optical semiconductor device includes a light emitting layer, a transparent layer, a first metal post, a second metal post and a sealing layer. The light emitting layer includes a first and a second major surface, a first and a second electrode. The second major surface is a surface opposite to the first major surface, and the first electrode and second electrodes are formed on the second major surface. The transparent layer is provided on the first major surface. The first metal post is provided on the first electrode. The second metal post is provided on the second electrode. The sealing layer is provided on the second major surface. The sealing layer covers a side surface of the light emitting layer and seals the first and second metal posts while leaving end portions of the first and second metal posts exposed.