Healing music and Musicotherapy attract attentions recently. They are beginning to be used in the medical treatment. On the other hand there are little practical utilization to relax with video images compared with music. Visual information is said to occupy about the whole of 80% humans receive from five senses. Therefore it has some potential that visual information affects human's mental and physical state as well as aural information does. This paper attempts to investigate the relation between characteristics of video images and mental relaxation. The one of characteristics we focus is motion speed of video images. It is considered that it might correspond to tempo of music. This paper investigates the relaxation effect by using the index calculated by heart rate variability analysis. The experimental results and analysis confirmed that viewing video images might have higher effect of reducing stress than eyes closed state and the relaxation effect might depend on motion speed.