1. An image dynamic range compression system comprising: an image converting unit configured to convert an input image into a compressed image which dynamic range is narrower than that of the input image; a high-frequency image extracting unit which configured to extract a high-frequency image from the input image; and an image synthesizing unit which configured to synthesize the compressed image and the high-frequency image, a registration unit configured to generate a high-frequency image that is registered to a high-frequency image extracted from the input image and another high-frequency image extracted from an image obtained by capturing an image of a same subject as the input image, wherein, the image synthesizing unit is further configured to synthesize the images by adaptively changing a synthesizing method of ensuring a visibility of images to synthesize, the registration unit is further configured to generate the high-frequency image that is registered to the high-frequency image extracted from the input image and an image obtained by geometrically converting the another high-frequency image, the image synthesizing unit is further configured to synthesize the compressed image and the registered high-frequency image, the registration unit is further configured to calculate registration reliability representing a degree of accuracy of registration for the generated high-frequency image, and the image synthesizing unit is further configured to change the synthesizing method for the compressed image and the high-frequency image according to the registration reliability.