To prevent jitters caused on a borderline, with which object video images are in contact when the frame rates of the object video images differ from each other.

A duplicate area detection section 158 detects a duplicate area 200 from contour data A108, B116, resulting from extracting areas of the object video images. However, by applying the contour data A108, B116 to a motion detection section 8, a motion of a borderline 2 of the duplicate area 200 is detected, and a duplicate area correction section 12 reserves a concealed area with a margin from the borderline 2 of the lower object video images, detected by the duplicate area detection section 158 toward the rear side of the video images, on the basis of the result of detection. Cutting the object video image by the borderline moved newly prevents the occurrence of jitters caused on the borderline 2.