1. An image processing method which applies image processing including gradation conversion processing to image data, the image processing method comprising: applying image processing to the image data based on each of a plurality of different gradation conversion processing characteristics, thereby generating a plurality of image-processed image data; calculating synthesis ratios of the plurality of image-processed image data by referencing image information of the image data prior to image processing; and synthesizing the plurality of image-processed image data based on the synthesis ratios, thereby generating image data to be outputted.