To exactly reconstitute an image in a short time by calculating the image from parallel projection formed by arranging and interpolating data supplied from a detector again from a fan-shaped geometrical position into parallel arrangement separately for each detector line.

A measuring device composed of an X-ray beam generator 9 and a detector 2 is rotated around a device axis 6, a detector signal is supplied to a computer 7, and the image showing the capacity of a patient to be examined is calculated and reproduced on a monitor 8. In this case, the parallel projection having channels as many as the case of fan-shaped geometrical arrangement is calculated and weight functions belonging to the parallel projection are provided and added so that the average value of parallel projection having weight to change beam direction and channel direction can be provided. Then, the weight function is approximated by the number of Fourier classes and a spectrum is interpolated to an orthogonal coordinate raster again and inversely transformed to a local space by a two-dimensional inverse FFT.