An image synthesis apparatus stores a synthesis order in which input images are to be synthesized with each other, and first and second synthesis numbers respectively designating how many of the input images are to be synthesized to generate first and second output images. The image synthesis apparatus synthesizes the plurality of input images with each other sequentially in accordance with the synthesis order, and stores a resultant synthesized image. When the stored synthesized image is the first output image, generated by synthesizing the first synthesis number of input images in accordance with the synthesis order, the first output image is output to an external destination. When the stored synthesized image is the second output image, generated by synthesizing the first output image with the remaining input images in the synthesis order, the stored second output image is output to the external destination.