A binarization method of binarizing an image by extracting lightness (brightness, density) as a feature amount from the image. When a pixel is selected in an image, a sensitivity is added to and/or subtracted from the value concerning the Y value of the selected pixel to set a threshold value range. Next, when another pixel is selected, the sensitivity is added to or subtracted from the value concerning the Y value of the selected pixel and a new threshold value range is set containing the calculation result and the already setup threshold value range. The pixel with the value concerning the Y value of any pixel in the image within the threshold value range is extracted as the same brightness as the selected pixel and the extraction result is displayed.