Abstract: According to one embodiment, a data storage device includes a data recording medium, a light source, and following units. The light application unit splits the laser beam from the light source, and applies the first and second light beams to the data recording medium from different directions. The light detection unit detects reflected light beams from the data recording medium. The light deflection unit deflects the reflected light beams to direct the reflected light beams to the light detection unit. The arithmetic unit calculates positional error information based on the detection signal. The drive unit displaces a position and a posture of the data recording medium based on the positional error information.

Abstract: Two light flux areas are disposed in a direction along which a pair of vertically opposite angles defined by first and second straight lines are aligned, and the other two light flux areas are disposed in a direction along which the other pair of vertically opposite angles are aligned. A spectral element sets propagating directions of divided elements of each of light fluxes obtained by dividing each of the light fluxes by a third straight line intersecting with the first and second straight lines by an angle of 45 degrees, or by a fourth straight line orthogonal to the third straight line to disperse the divided elements on a photodetector. The photodetector is provided with sensors which individually receive the divided elements of the light fluxes.