An optical semiconductor device comprising an emitted beam branching
section (61) which branches an emitted light beam from a laser device
(51), a reflected light beam branching section (71) which branches a
reflected light beam from an information recording medium (3) into light
beams different from each other in focused state, servo signal sensing
photodetectors (43, 45) which receive the branched reflected light beam
in a defocused state, a first diffraction grating provided in the emitted
light beam branching section for diffracting the reflected light beam
having passed through the reflected light beam flux branching section,
and a signal sensing photodetector (47) which receives the reflected
light beam diffracted by the first diffraction grating.