A digital still camera records image data by picking up an image of an object. One mode is set among plural photographing modes including a standard photographing mode and a portrait mode adapted to an image containing an abrupt change in gradation. An optimized exposure amount is acquired according to brightness derived from the image data. When the portrait mode is set, portrait mode image data is acquired by setting exposure lower than the optimized exposure amount. The image data is processed in gradation conversion by use of first or second gamma conversion table. The first gamma conversion table is used for the standard photographing mode. The second gamma conversion table is used for the portrait mode, predetermined to extend a dynamic range of the image data after the gradation conversion in comparison with the first gamma conversion table, to process the portrait mode image data in gradation conversion.