1. A video terminal, comprising: a display image former that generates a display image including a first image that is a moving image and a second image belonging to a higher layer than the first image, the second image being superimposed on the first image; and an update rate adjuster that determines an update rate of the first image in the display image based on a comparison between a first exposure area of the first image and a first threshold, the first exposure area is a number of pixels of an area not covered by the second image within an insertion area of the first image in the display image when semi-transparent combining processing has not been performed, wherein the update rate adjuster determines the update rate based on a comparison between a second exposure area and a second threshold, the second exposure area is a sum of products of opacity and brightness in each unit area within the insertion area when semi-transparent combining processing has been performed.