PURPOSE:To eliminate spherical aberration by constituting a focus error detecting element so that the first surface is a toric surface having positive power and the second surface is the surface of rotation symmetry having negative power sequentially from the side upon which a light beam is incident. CONSTITUTION:When the focus position of an objective lens is not present on an information substrate, a light beam 1 which is incident upon a focus error detecting element 2 becomes slightly divergent light or convergent light, so that the position at which the light beams 1 is focused is displaced. In this case, a pattern on a light-intercepting element is changed in accordance with the displacement of focus. The change of a pattern is received by a four-division light-intercepting element 8, thereby being able to detect a focus error signal. The aspheric form 4 on the second surface side determines the compensation for spherical aberration on a focal point 7 relative to the spheric form 5 on the first surface side. Next, the aspheric form 3 on the first surface side determines compensation for spherical aberration on a focal point 6 in combination with the aspheric form 4 on the second surface side, thereby compensating for spherical aberration on the focal points 6 and 7.
FOCUS ERROR DETECTING ELEMENT AND OPTICAL HEAD USING THE SAME