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; and a second synthesizing unit configured to, when an image of another frame is to be synthesized with the first synthesized image, 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 other frame to the subtracting result to generate a synthesized image.