An optical semiconductor device comprises a semiconductor laser mounted on a bottom portion of a package, a receiving unit that receives a signal light from an outside using local oscillator light output from the laser, and a receiving-unit-mounted substrate on which the receiving unit including a semiconductor light receiving element is mounted. The receiving unit is disposed on a surface on the side opposite to the laser. The light receiving-unit-mounted substrate is a non-transmissive substrate, has a light passing portion through which the local oscillator light output from the laser passes, and covers the bottom portion surrounded by an outer peripheral portion of the package.