A tracking servo system for an optical-disc information reproducing apparatus includes (a) a light source for generating a light beam to scan a target track on an optical-disc wherein the information is stored as a series of pits arranged in the form of the track; (b) four photodetectors, arranged in two pairs with each pair intersecting the other and being disposed in a direction oblique to an image of the track for converting an image of the pit into respective electric signals; (c) a circuit for comparing the phases of the electric signals and for producing a tracking error signal ranging in accordance with the phase difference between the electric signals; and (d) a motor and lens for deflecting the light beam along a path perpendicular to the target track in response to the tracking error signal, whereby the light beam accurately scans the target track.