Patent ID: 7821902

Claim:
An aberration detection device comprising: light beam dividing means which divides a bundle of light beams passing through a condensing optical system and reflected from an information storage medium into first light beams and second light beams, the first light beams including an optical axis of the bundle of light beams, the second light beams not including the optical axis; light detecting means having a plurality of light receiving sections which separately receive the first light beams and the second light beams, which are obtained by the light beam dividing means; and spherical aberration detecting means which detects spherical aberration of the condensing optical system on the basis of the amount of the first light beams received by the light receiving sections and the amount of the second light beams received by the light receiving sections, wherein the light beam dividing means has a first border and a second border, the first border extending in a radial direction and passing through the optical axis, the second border having segments at least both ends thereof and a bulge in a center thereof, the segments being substantially parallel to the first border, the bulge bulging toward a periphery of the light beam dividing means so that a top of the bulge is substantially parallel to the first border.