In a cut mask preparation method of displayed image on a screen of display means to prepare a cutting mask, an area lying across a real region and a background region is designated as a training area, for picture elements in which area a YMC three plate histogram is prepared to determine a threshold through discrimination analysis. A plate among the three plates, which has a maximum estimating function used upon the threshold determination, is used and binary-coded with said threshold to prepare a mask for a picture element included in one between the binary-coded values.