Optical head device

An optical head device comprising:

• a diffraction element diffracting a light beam reflected from an information medium and being divided into a plurality of areas in a direction of a radius of a luminous flux of the light beam;
• means for dividing the diffracted light beam;
• a converging optical system converging the divided diffracted light beam;
• and
• a photodetector having a plurality of photodetection areas and detecting the converged diffracted light beam,
• wherein (1) a distance between a first position, in a plurality of photodetection areas, on which a principal ray of a divided diffracted light beam from an area, close to a center of an optical axis of the light beam, of a plurality of areas of said diffraction element is incident and a position of a boundary line between the photodetection area on which the divided principal ray is incident and another photodetection area adjoining the area is smaller than a radius of a minimum spot of a diffracted light beam that could be formed on said photodetector if the divided diffracted light beam were not divided, and (2) a distance between a second position, in a plurality of photodetection areas, on which a principal ray of a divided diffracted light beam from an area, far from the center of the optical axis of the light beam, of a plurality of areas of said diffraction element is incident and a position of a boundary line between the photodetection area on which the divided principal ray is incident and another photodetection area adjoining the area is equal to or larger than the radius of the minimum spot of the diffracted light beam that could be formed on said photodetector if the divided diffracted light beam were not divided.