PURPOSE: To efficiently irradiate a light output from a light emitting source by arranging a Faraday rotating element between an information recording medium and a polarized beam splitter and providing a light isolator made of the 2nd polarized beam splitter and a Faraday rotating element.

CONSTITUTION: A light beam emitted from the light emitting source 1 is collimated into a parallel beam by a collimate lens 2, and makes incident on the 2nd polarized beam splitter 13. The incident light beam passes through the splitter 12 without being hardly damaged. The light beam passing through the polarized beam splitter 13 makes incident on the Faraday rotating element 14, and the polarization orientation angle of a transmitting beam is rotated by 45° against an advancing direction. After passing through a 1/2 wavelength plate 16, the light beam whose polarization orientation again points the direction of an axis (x) makes incident on the 1st polarized beam splitter 17, and passes through said splitter without being hardly damaged. Finally, the light beam passing through the 1st polarized beam splitter 17 is rotated by the Faraday rotating element 18, after which it is condensed and irradiated on the information recording medium 6 by an objective lens 5.