The present invention is directed to a digital still camera (1) using CCD (Charge Coupled Device) image sensor as image pick-up device. This camera (1) comprises an image memory (18) in which plural images obtained by picking up images of the same object by different exposure values are stored, an image data adding circuit (17) for synthesizing the plural images stored in the image memory (18) to generate synthetic image in which dynamic range is broad, a Y gradation correction circuit (24) and a multiplier circuit (26) for compressing gradation of the synthetic image generated by the image data adding circuit (17), and a controller (29) for generating gradation conversion table for carrying out gradation compression. The controller (29) generates gradation conversion table on the basis of synthetic curve in which line determined on the basis of ratio between the number of gradations of synthetic image and the number of gradations of output image and histogram indicating luminance distribution of synthetic image are synthesized.