1. An arithmetic device comprising: a projection data creation unit that creates object projection data on the basis of information regarding X-rays which are irradiated from respective directions around an object and are transmitted through the object; a reference image creation unit that creates a reference image on the basis of the object projection data; a standardized image creation unit that creates a standardized image in which a standardization coefficient for adjusting the noise reduction intensity in an update process is defined for each pixel; and a repetitive processing unit that performs repetitive processes a predetermined number of times by using the reference image and the standardized image, wherein the repetitive processing unit includes a difference image creation portion that obtains a difference between the reference image and an updated image which is obtained through the update process, so as to create a difference image, and an image update portion that performs the update process by using the difference image and the standardized image, so as to create an updated image.