PURPOSE: To obtain a focus error signal by providing a truncated conical optical element on the light emission side of a polarizing prism which reflects reflected light from a disk, and by detecting variations in the quantities of reflected light and transmitted light on the slanting surface of the optical element.

CONSTITUTION: On the light emission side of a polarizing prism 3, an optical detecting element 30 having a slanting surface 30a set nearly at a critical angle to an incident light beam in a focusing state is provided. In the focusing state, the beam is totally reflected by the slanting surface 30a to be transmitted in the element by being refracted by the truncated surface 30b and when a disk 6 shifts as shown by an arrow (a), it is incident at an angle less than the critical angle to be separated into a reflected part and a refracted and transmitted part while a (b) directional shif increases an incidence angle greater above the critical angle to cause total reflection, so that it is refracted by the truncated surface 30b and transmitted. Photodetecting elements 32AW32C are provided opposing to the truncated surface 30b to obtain a focus error signal by using the difference between the sum of the outputs of the 32A and 30B, and that of the 32C.