Provided is an apparatus for continuously nitriding a strip continuously being fed after cold rolling and before secondary recrystallization annealing in a production line of a grain-oriented electrical steel sheet, comprising: a nitriding zone for nitriding the strip; a cooling zone for cooling the strip; and an optional heating zone provided upstream of the nitriding zone for heating the strip, wherein, the nitriding zone is provided with glow discharge electrodes, and by plasma nitriding the strip by glow discharge with the glow discharge electrodes functioning as positive electrodes and the strip functioning as a negative electrode, inhibitor forming elements are uniformly dispersed over the full length and full width of the strip and a grain-oriented electrical steel sheet with excellent magnetic properties with no variation is obtained.