1. An image synthesizing apparatus that synthesizes a plurality of frames to obtain a synthesized image, comprising: a first synthesizing unit configured to synthesize a preset number of frames to obtain a first synthesized image, wherein the first synthesized image is stored in a memory; and a second synthesizing unit configured to, when an image of an alternative frame is to be synthesized with the first synthesized image, read out the first synthesized image from the memory, subtract an image of a predetermined frame used for the synthesis obtaining the first synthesized image from the first synthesized image, and add the image of the alternative frame to the subtracting result to generate a third synthesized image.