Image display device and gradation change method

[Problem] To provide an image display device that, when displaying on a screen an electronic document, image or the like having a watermark display, is capable of reducing the watermark display without altering the electronic document or the like. [Solution] The image display device is provided with a gradation change means (2) capable of changing an input gradation from an image input signal into output gradation values different for each input gradation value, and a control circuit (7) that controls various signals. The image display device is configured so that in order to reduce the watermark display in the electronic document, image or the like and then to display the electronic document, image or the like, the output gradation value corresponding to the watermark gradation value, out of output gradation values for the gradation change means (2), is changed to a value either the same or close to the background output gradation value.