1. An imaging method in an industrial X-ray CT apparatus for transmitting an X-ray emitted from an X-ray source through a measurement object including two substances having different attenuation factors, detecting intensity of X-ray transmitted through the measurement object by a detector and constructing a three-dimensional image from a plurality of CT images, each obtained in a slice surface of the measurement object, comprising the steps of: setting a position for acquiring a CT image in a first slice surface corresponding to a boundary surface of said two substances having different attenuation factors; moving said X-ray source and said detector to said first slice surface and two or more upper and lower heights in a height direction perpendicular to said first slice surface and detecting intensity of X-ray by said detector at said two or more upper and lower heights of said first slice surface for each CT image to be imaged; obtaining a rate of intensity change of X-ray in said height direction on the basis of the intensity of X-ray measured at said two or more upper and lower heights of said first slice surface; correcting the intensity of X-ray measured at said first slice surface on the basis of said rate of intensity change of X-ray; and obtaining a CT image in said first slice surface on the basis of intensity of X-ray corrected.