An X-ray CT system comprising: an X-ray tube configured to irradiate an X-ray; an X-ray detector situated so as to face the X-ray tube and configured to detect the X-ray having passed through an object; a scan controller configured to scan the object by rotating the X-ray tube and the X-ray detector around the object in order to obtain X-ray transmission data by means of the X-ray tube and the X-ray detector; an extracting part configured to extract the X-ray transmission data of two specific directions orthogonal to each other from the X-ray transmission data to which a fan-parallel conversion is applied; an outline calculator configured to calculate an outline of the object from the X-ray transmission data obtained by a full scan, which is a scan in which the X-ray tube and X-ray detector are rotated around the object to irradiate an X-ray at each angle in a direction, said outline calculator configured to calculate the outline of the object at a tomographic position represented by the X-ray transmission data of the specific direction, based on the X-ray transmission data of the specific direction; and a reconstructing part configured to reconstruct image data of an inside of the object based on corrected X-ray transmission data, and comprising a correction part adapted to correct for scattered radiation components, and/or an effect of beam hardening, based on the outline calculated by the outline calculator, said corrected X-ray transmission data being corrected by the correction part, wherein the outline calculator includes a converter configured to execute fan-parallel conversion on the X-ray transmission data obtained by the scan controller, a calculator configured to calculate the outline of the object by calculating a major diameter, a minor diameter, a shift amount in a major diameter direction, and a shift amount in a minor diameter direction of the object based on the X-ray transmission data of the two specific directions, and an interpolating part configured to interpolate, based on the outline of the object at the tomographic position represented by the X-ray transmission data of the specific direction, the outline at a position other than the tomographic position to obtain an interpolated value of the major diameter, minor diameter, and shift amount at a tomographic position of a different direction other than the specific direction, wherein said interpolated value at said direction is obtained by forming a weighted sum of said major diameter, said minor diameter, a shift amount in a major diameter direction or a shift amount in a minor diameter direction of the object at said two specific directions, and wherein the weighted sum is a weighting according to a ratio of the distances in the axis direction of said two specific directions. an X-ray tube configured to irradiate an X-ray; an X-ray detector situated so as to face the X-ray tube and configured to detect the X-ray having passed through an object; a scan controller configured to scan the object by rotating the X-ray tube and the X-ray detector around the object in order to obtain X-ray transmission data by means of the X-ray tube and the X-ray detector; an extracting part configured to extract the X-ray transmission data of two specific directions orthogonal to each other from the X-ray transmission data to which a fan-parallel conversion is applied; an outline calculator configured to calculate an outline of the object from the X-ray transmission data obtained by a full scan, which is a scan in which the X-ray tube and X-ray detector are rotated around the object to irradiate an X-ray at each angle in a direction, said outline calculator configured to calculate the outline of the object at a tomographic position represented by the X-ray transmission data of the specific direction, based on the X-ray transmission data of the specific direction; and a reconstructing part configured to reconstruct image data of an inside of the object based on corrected X-ray transmission data, and comprising a correction part adapted to correct for scattered radiation components, and/or an effect of beam hardening, based on the outline calculated by the outline calculator, said corrected X-ray transmission data being corrected by the correction part, wherein the outline calculator includes a converter configured to execute fan-parallel conversion on the X-ray transmission data obtained by the scan controller, a calculator configured to calculate the outline of the object by calculating a major diameter, a minor diameter, a shift amount in a major diameter direction, and a shift amount in a minor diameter direction of the object based on the X-ray transmission data of the two specific directions, and an interpolating part configured to interpolate, based on the outline of the object at the tomographic position represented by the X-ray transmission data of the specific direction, the outline at a position other than the tomographic position to obtain an interpolated value of the major diameter, minor diameter, and shift amount at a tomographic position of a different direction other than the specific direction, wherein said interpolated value at said direction is obtained by forming a weighted sum of said major diameter, said minor diameter, a shift amount in a major diameter direction or a shift amount in a minor diameter direction of the object at said two specific directions, and wherein the weighted sum is a weighting according to a ratio of the distances in the axis direction of said two specific directions.