PURPOSE: To decrease the number of component parts and to reduce the cost of a device, by unifying a parallel plane plate with a polarized beam splitter to obtain a astigmatism function.

CONSTITUTION: The light delivered from a semiconductor laser light source 1 is turned into parallel beams by a cemented lens 2 and then mase incident to a parallel plane plate 10 having both astigmatism application function and polarized beam splitter function as well as an incident surface tilted by 45°. Then this reflected light is condensed on the surface of a disk 6 through a λ/4 plate 4 and an objective lens 5. This reflected light travels backward to pass through the plate 10 and is focused onto a 4-split photodetecting element with astigmatism by a condenser lens 7. Thus both focus detection and tracking are carried out. As a result, the number of component parts is decreased along with a cost reduction of a device.