An image processing unit includes a memory unit, a comparative image generator to generate a comparative image from a non-reference image, an image divider to divide a reference image, a non-reference image, and a comparative image into image blocks of a predetermined size, a motion data calculator to calculate motion data between the reference image and comparative image, an average calculator to calculate an average of pixel output values for each image block of the reference image and the comparative image, a threshold determiner to determine a threshold for synthesis determination according to the average of pixel output values of an image block of the reference image, a synthesis determiner to determine whether or not the image blocks of the reference image and of the non-reference image are suitable for synthesis, and a ratio determiner to determine a synthesis ratio of the image blocks determined as suitable for synthesis.