Disclosed is a focus detection apparatus for a projection lithography system. The apparatus includes: a laser; a focus optical unit configured to focusing the emitted laser beam; a force detection unit configured to reflect the focused laser beam at the backside; a position detection unit configured to detect variations in position of a light spot formed by the reflected laser beam, and output a strength signal indicating the strength of the interaction force between the force detection unit and the object; a differential amplifier configured to output a Z-direction differential signal based on the strength signal and a reference signal; a Z-direction feedback control unit configured to perform feedback control; and a scan signal generator configured to output a signal for controlling the movement of the stage in the XY plane. The focus detection apparatus has high precision, efficiency and process applicability.