A corrector corrects a main image acquired by an image acquirer based on correction setting acquired by a correction setting acquirer, and stores corrected main image in a buffer. An unused area detector detects the area that has not stored the corrected main image in the buffer, and transmits the result to a sub-image selector. The sub-image selector selects, among sub-image candidates registered in a sub-image information memory, a candidate that can be displayed on the unused area and has the highest priority as a sub-image. The sub-image controller executes a program stored in the sub-image displaying program memory, and displays the selected sub-image on the unused area.