A method for generating approximate functions of total MTFs of x-ray images, based on conditions for imaging with an x-ray emitted by an x-ray tube, includes: a first generating step of generating an approximate function of an MTF based on an x-ray tube effective focal spot value on an object plane； a second generating step of generating an approximate function of an MTF based on the kind of an x-ray detector system on the object plane； a third generating step of generating an approximate function of an MTF based on the amount of motion of an object； and a fourth generating step of generating an approximate function of a total MTF.
