Disclosed is an image synthesizer including an image frames memory which memorizes a plurality of image frames which are produced by continuously taking pictures of a subject, an image synthesizer which synthesizes a plurality of image frames to a summation image, an image brightness adjuster which adjusts brightness of the summation image by synthesizing the image frames with the image synthesizer, and a display which displays the summation image which is being synthesized by the image synthesizing means in the image brightness adjusting processing.