Opening/closing device for vehicle sliding-window panel

An opening/closing device for a vehicle sliding-window panel, includes a sliding-window panel opening and closing an opening part provided in a vehicle body, upper and lower guide rails to support the sliding-window panel to slide, and a slide driving mechanism fixed to one guide rail either the upper or lower guide rail. The slide driving mechanism includes a motor, a lead screw rotated by driving the motor, and a screw nut which is screwed with the lead screw to linearly move in a longitudinal direction of the one guide rail by rotation of the lead screw and is coupled with the sliding-window panel to operate the sliding-window panel to open and close by the linear movement. The one guide rail is integrally formed with a first and second supporting portions which rotatably support both end portions of the lead screw respectively.