A computing device configured to obtain information about a subject using a detection result detected by an X-ray detector which detects an X-ray passing through the subject, which device includes: a unit configured to obtain a detection result of the X-ray detector; a first obtaining unit configured to obtain a complex refractive index of the X-ray after passing through the subject using the detection result; and a second obtaining unit configured to obtain information about the subject in accordance with a correlation between the complex refractive index and a mass absorption coefficient.