Abstract: A focus error detector for detecting a focus error of an objective lens with respect to an optical disk. The focus error detector includes a beam splitter for splitting light reflected from the optical disk. Two critical angle prisms having critical angle reflecting surfaces are included for respectively reflecting the light rays from the beam splitter at the respective critical angles onto two bisectional photo-detectors. The bisectional photo-detector generates detecting signals responsive to the amount of light received. The detecting signals are differentiated and added in a direction offsetting a shift or tilt of the reflected light. Accordingly, only the focus error signal is detected, irrespective of the tilt and shift of the optical axis of the reflected light. Thus, malfunction of an optical pickup is prevented, thereby providing for the uncorrupted recording and reproduction of data.