1. An inner defect inspection method for inspecting inner defects in an object of casting parts on the basis of X-ray CT cell values obtained by imaging said object of casting parts by an X-ray CT apparatus, said X-ray CT cell values corresponding to density of said object, said inner defect inspection method comprising the steps of: extracting an inner defect from X-ray CT data of the object of casting parts by using inner defect extraction means; collecting a plurality of X-ray CT cells included in a neighborhood range, which is set with twice to three times the number of X-ray CT cells included in the inner defect around the inner defect extracted by the inner defect extraction means, as related X-ray CT cells by using related element collection means from the X-ray CT data of the object of casting parts; measuring feature quantities of the inner defect on the basis of the related elements collected by the related X-ray CT cells collection means, by using feature quantity measurement means; and displaying inner defects whose measured feature quantities satisfy a predetermined threshold on a display.