PURPOSE: To separate an overwrite spot and a verify spot in a desired light quantity ratio by inclining the optical axis of a third approximately rectangular prism at 45° to the direction of polarization of the luminous flux emitted from the third end face of a second rectangular prism and making the optical axis of the third prism orthogonal to that of a fourth approximately rectangular prism.

CONSTITUTION: Light is separated to the overwrite beam and the verify beam in directions of polarization orthogonal to each other in a prescribed light quantity ratio by a uniaxial crystal prism 16-1 and a glass prism 16-3, and they are made incident on a magneto-optical disk. Respective beams which are condensed on the disk and are reflected pass the prism 16-3 and are allowed to pass a uniaxial crystal rectangular prism 16-4, which has the optical axis inclined at 45° to the direction of polarization of the incident beam to the magneto-optical disk in a plane perpendicular to the reflected beam, and a uniaxial crystal rectangular prism 16-5, which is joined to the prism 16-4 and has the optical axis orthogonal to the prism 16-4, and are separated to four beams whose directions of polarization are orthogonal to one another. Thus, photomagnetic signals of the overwrite beam and the verify beam are obtained.