1. An image display device, comprising: a region of interest extraction unit that i) generates a depth image signal by depth image conversion employing a depth threshold generated by an amount of movement of images in a 2D/3D mixed display, the depth image signal including information on a distance in a three-dimensional space between a viewpoint and each pixel of a two-dimensional image including a region of interest desired to be noted by an observer, the depth image conversion being such that a depth value for each pixel, the depth value indicating the distance between the viewpoint and each pixel of the two-dimensional image is converted to a new depth value for 2D display when the depth value is equal to or larger than the depth threshold, and ii) extracts the region of interest based on the generated depth image signal; a parallax image generation unit that generates a left- and right-eye parallax images each having a parallax in a new region of interest resulting from the new depth value and having no parallax in a region other than the new region of interest, from said two-dimensional image and an image obtained by conversion of a region of interest image to be displayed in the region of interest at each of right and left viewpoints, based on said two-dimensional image and said depth image signal; and a 3D image display unit that displays the left-eye parallax image and the right-eye parallax image, wherein the region of interest extraction unit comprises an image segmentation unit that receives a first two-dimensional image of a most recent frame and a second two-dimensional image of a frame immediately preceding the most recent frame of the first two-dimensional image, divides each of the first and second two-dimensional images into divided regions each having common characteristic(s) to the first and second two-dimensional images, and generates a divided first two-dimensional image and a divided second two-dimensional image, an optical flow computing unit that computes a difference value between center positions of gravity of a divided region of the divided first two-dimensional image and a divided region of the divided second two-dimensional image, as an optical flow of the divided region of the divided first two-dimensional image, a depth image computing unit that generates a depth estimation value for each pixel of the divided region of the divided first two-dimensional image based on luminance information on the divided region of the divided first two-dimensional image, and a depth image conversion unit that computes the depth threshold from the optical flow, thereby performing the depth image conversion on the depth estimation value, and wherein the depth threshold is computed by multiplying the optical flow by a proportionality constant and then adding to the multiplied optical flow a predetermined constant indicating a depth value when the optical flow is zero.