1. A radiographic apparatus for obtaining images of sectional planes in a region of interest of an object under examination by an image reconstruction using projection data acquired by radiographing the object from varied scan positions, said apparatus comprising: a radiation source for irradiating said object with penetrating electromagnetic waves; an area detector opposed to said radiation source across said object for detecting electromagnetic waves transmitted through said object; scanning means for synchronously moving said radiation source and said area detector for scanning action; and a back projection unit for performing an image reconstruction to generate three-dimensional volume data of the region of interest by projecting projection data detected in the varied scan positions back to predetermined lattice points of a three-dimensional lattice virtually set to the region of interest of the object radiographed.