1. A stereo image measuring apparatus for respectively displaying left and right stereo images on monitors so that they are three-dimensionally viewed, comprising: a pair of memories for storing left and right stereo image data therein respectively; a pair of monitors each having first and second display regions and for three-dimensionally displaying at least two kinds of images thereon respectively; a pair of main image memories for storing therein image data for respectively displaying the left and right stereo images on said first display regions of said monitors at a first magnification; first transfer means for transferring under compression the left and right stereo image data from said memories so as to be displayed on said first display regions of said monitors at the first magnification; a pair of sub image memories for storing therein image data for respectively displaying the left and right stereo images on said second display regions of said monitors at a second magnification; a cursor for designating measuring points onto a main image displayed on each of said first display regions; and second transfer means for transferring image data adjacent to measuring points to each of said sub image memories so as to be displayed on each of said monitors at the second magnification larger than said first magnification based on the left and right stereo image data stored in said memories when the measuring points are designated on the main image displayed on each of said first display regions.