An optical pickup head apparatus comprising: a light source; an imaging optics for receiving a linearly polarized light beam emitted from the light source, and converges it on a small spot on an information medium; photodetector units for receiving the light beam being reflected and diffracted at the information medium and output electric signals corresponding to received light quantities; a liquid crystal hologram consisting of a pair of transparent substrates, a liquid crystal sandwiched therebetween through a pair of transparent electrodes, either of the transparent electrodes being formed with a hologram pattern; and a Faraday rotator for rotating a polarized direction of light beam by 45 degrees in a light path leading from the light source to the information medium and, further, rotates the polarized direction of the light beam by 45 degrees in its opposite light path. The liquid crystal hologram is applied a constant voltage so that the light beam reflected at the information medium is diffracted in the liquid crystal hologram to generate .+-.1-order diffraction light beams, and the photodetector units sense both of light quantities of said .+-.1-order diffraction light beams emitted from the liquid crystal hologram. Then, a subtraction between signals representing the light quantities is calculated so as to obtain an information signal.