1. An image processing apparatus comprising: a gray-scale conversion characteristic deriving unit configured to set reference image data out of a plurality of input image data obtained by shooting an identical object with a different amount of exposure and derive a gray-scale conversion characteristic from the reference image data; and an image synthesis processing unit configured to derive a new pixel value for each pixel using a pixel value of one or more input image data selected from a plurality of the input image data based on the gray-scale conversion characteristic to create synthesized image data; wherein the image synthesis processing unit is configured to derive an amplification factor for each pixel value of the reference image data using the gray-scale conversion characteristic, select input image data obtained with an amount of exposure corresponding to a magnitude of the amplification factor, and derive the new pixel value using the pixel value of the selected input image data.