A light source beam is incident on an optical guide block and converted in its diffusion angle by a diffusion angle conversion film on a second inclined portion of the optical guide block, and reflected by a reflecting film on a first inclined portion. Then, the light is incident on a first beam splitter film on the second inclined portion and split into transmitted light and reflected light. The reflected light is focused an optical disk light reflected from the disc passes through the first beam splitter film, and then is transmitted and reflected by a second beam splitter film on a third inclined portion. This transmitted light is incident on a polarization plane conversion plate between the third inclined portion and a fourth inclined portion, so that P and S polarization components and are transmitted through and reflected by its polarized light splitting portion, respectively. The S polarization component is reflected by its reflecting surface toward light-receiving elements. The reflected light from the second beam splitter film is reflected by a focus error detection element on the second inclined portion, and an incident light-splitting point of the polarized light splitting portion is located between the center of the optical guide block and the light-receiving elements, thereby reducing a light amount loss and also increasing a Kerr rotation angle so that an RF reproduction signal with a high C/N ratio can be obtained.