1. An X-ray attenuation correction method comprising steps of: extracting boundary information including boundary positional information of a boundary position where X-ray absorption rate is changing and magnitude information indicative of magnitude of said change, by using X-ray projection information of a subject or tomographic image information being generated by image reconstruction of said projection information; and correcting X-ray attenuation of said boundary position included in said projection information or said tomographic image information by using said boundary information.