A thin window regulator which can be assembled easily and which requires less installation space inside a door panel is disclosed. Both longitudinal ends of a belt are connected to a pair of sliders and the belt is made into a ring shape. Sliders are movable along a guide rail and are biased closer to each other by springs. Thus, since the ring-shaped belt is wound around a sprocket and the sliding portion, tension is not reduced by a creep phenomenon and the belt does not loosen. One end of the ring shaped belt is formed to be wound around the sliding portion. Thus, the window regulator can be made thinner compared with prior art systems in which both ends of a belt are wound around pulleys and a required installation space inside the door at the upper portion of the guide rail can be made thinner.