An optical data recording device is suitable for optical storage medium. An optical read-write head transmits light beam to a recording region of optical storage medium and receives the light beam reflected by the recording region of optical storage medium. A sled mechanism moves the optical pickup unit in a predetermined direction. A processor obtains a boundary between a recorded region and an unrecorded region on the optical storage medium according to the reflected light beam, and obtains a recording address corresponding to the unrecorded region next to the boundary. According to the invention the boundary between the recorded region and the unrecorded region of the optical storage medium can be detected without reading the address information of the optical storage medium, and the speed for detecting the boundary is increased.