A sliding nozzle device automatically performs a series of operations of loading and unloading pressure between plates and opening and closing a slide frame; maintaining the pressure without additional operations; and operates at full stroke during molten steel flow control. An auxiliary plate-exchanging mechanism includes slide axes moving in the same direction as a hydraulic cylinder operates, and an arm having a proximal end placed around the slide axis. The plate-exchanging mechanism is fixed on an upside frame. A first engagement pin mounted on the proximal end of the arm is inserted in a first engagement groove in a first engagement member engaging with the slide axis, and second engagement pins mounted on bearings are inserted in second engagement grooves in the slide axes. With movement of the slide axes the engagement pins respectively move in the engagement grooves, thereby rotating the slide axes and the arm.