1. An image processing device for generating a synthesized image from a plurality of continuous still images which are obtained by a continuous shooting, the image processing device comprising: a synthesizing unit operable to generate the synthesized image by synthesizing the still images; and a calculating unit operable to calculate a synthesized shutter speed indicating a shutter speed of the synthesized image after the synthesizing, by multiplying a continuous shooting shutter speed by a number of pieces of still image data that are used to generate the synthesized image.