A method for decreasing the streak artifact in the reconstructed image due to the existence of foreign matter of high X-ray absorption rate such as metals. The method includes separating discrete region containing the data having abrupt change of value the projection data in the sequential order of radiographic detector elements, transforming the data to continuous data such that the value gradually smoothly changes, filtering the continuous data, composing the filtered data with the data of the discrete regions to form the projection data used for the image reconstruction.