An optical semiconductor device includes a light emitting element having a first surface and a second surface, the first surface having a first electrode provided thereon, the second surface being located on the opposite side from the first surface and having a second electrode provided thereon; a first conductive member connected to the first surface; a second conductive member connected to the second surface; a first external electrode connected to the first conductive member; a second external electrode connected to the second conductive member; and an enclosure sealing the light emitting element, the first conductive member, and the second conductive member between the first external electrode and the second external electrode, and being configured to transmit light emitted from the light emitting element.