The influences of the vibrations and the like of a rotational radiography system are corrected with respect to a projection image for each projection direction which contains an artificial artifact, and volume data containing only a ring artifact is generated by image reconstruction using the projection images. A ring center is determined by using this volume data, and polar coordinate conversion of an actual radiographed image is executed by using the ring center as an origin, thereby extracting a signal (ring artifact) parallel to an angular axis.