1. An image pickup apparatus comprising: a picture-taking section to continuously take images of a subject to output a plurality of image frames; an image synthesizing section to synthesize the plurality of image frames to produce a synthesized image; an image brightness adjusting section which allows the image synthesizing section to synthesize the image frames and executes image brightness adjusting processing for adjusting a brightness of the synthesized image while the picture-taking section continuously takes the images of the subject; and a partway-image display section to display a partway synthesized image of the synthesized image, which is produced by the image synthesizing section in the image brightness adjusting processing executed by the image brightness adjusting section, wherein the image brightness adjusting section executes the image brightness adjusting processing such that an entire image of the synthesized image is gradually brightened as a number of the image frames synthesized by the image synthesizing section increases until the brightness of the synthesized image produced by the image synthesizing section becomes almost equal to a brightness of an image taken by a normal picture-taking method, and after the brightness of the synthesized image produced by the image synthesizing section becomes almost equal to the brightness of the image taken by the normal picture-taking method, the image brightness adjusting section executes the image brightness adjusting processing such that only a dark portion of the synthesized image is gradually brightened as the number of the image frames synthesized by the image synthesizing section increases.