In a window regulator, a positioner is provided between a pair of slider bases and a pair of windowpane holders and define a relative position therebetween in a forward/rearward direction of a vehicle; a vehicle-widthwise-directional wall is provided on each guide rail; and a pair of forward/rearward directional guide walls is provided on each slider base to hold an associated vehicle-widthwise-directional wall in the forward/rearward direction and to guide an associated slider base along an associated guide rail. A clearance in the forward/rearward direction between the vehicle-widthwise-directional wall of one guide rail and the forward/rearward-directional guide walls of one slider base is set smaller than that between the vehicle-widthwise-directional wall of the other guide rail and the forward/rearward-directional guide walls of the other slider base.