PURPOSE: To stabilize the intensity of the light entering a photodiode by subjecting the light incident on the photodiode to filtering at variable transmittivity by using a liquid crystal.

CONSTITUTION: A nematic liquid crystal material is used for a liquid crystal 15 and the molecules thereof are oriented with 90° twist by the interaction with the boundary of a substrate 12 and are thus made into a single domain. Polarizing plates 16 are disposed on the outside of the liquid crystal cell in such a way that the polarizing directions thereof cross each other. The polarizing directions coincide with the orientation of the liquid crystal molecules at the boundary of the substrate 12. The incident light is polarized by the plate 16 on the incident side and the polarized light is rotated along the twist of the liquid crystal molecules when the light is made incident on such liquid crystal device. The light transmits the analyzer as it is while no voltage is impressed to the cell. The liquid crystal molecules begin to move so as to parallel with the electric field direction and the initial twist structure begins to collapse when the voltage is impressed to the cell. The desired change of the quantity of the light entering the photodiode of the optical head is thus made possible if the voltage to be impressed to the liquid crystal 15 is set between V1 and V2.