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.