A method for estimating a scattered ray intensity distribution in an X-ray CT apparatus, the method includes: irradiating a subject with X-rays; and configuring a cross-sectional image of the subject by detecting the X-rays passing through the subject, on the basis of a path length of a scattered ray passing through the subject, an X-ray absorption coefficient of the subject and an X-ray scattering probability of the subject, intensity of the scattered ray being calculated.