PURPOSE: To enable an observer to engage solely in observation by displaying plural sets of image data successively on plural image displaying means and maintaining the displays.

CONSTITUTION: Many sets of the image data S inputted from the outside are transmitted sequentially by an image data transfer control means 2 to one or plural image displaying means 1a, 1b,...,1n in accordance with the prescribed display sequence. Further, an instruction operation is so made by an instruction operation means 3 as to change the prescribed display sequence. The need for the observer to assign the image data and the displaying means which display the visible images based thereon by each one is thus eliminated. In addition, the visible images displayed on the means 3 are held preserved as they are and the visible images based on the remaining image data can be sequentially displayed on the remaining image displaying means and, therefore, the observer is able to engage solely in the observation of the sequentially displayed visible images.