1. A radiographic apparatus for generating three-dimensional volume data of a region of interest of an object under examination by an image reconstruction of projection data acquired by radiographing the object from varied scan positions, and obtaining sectional images from the three-dimensional volume data, said apparatus comprising: a radiation source for irradiating said object with penetrating electromagnetic waves; an area detector for detecting electromagnetic waves transmitted through said object; said radiation source and said area detector being arranged across sectional planes of the object and synchronously operable for scanning action; and a back projection unit for performing the 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; said back projection unit generating the three-dimensional volume data, with lattice spacing along a sectional axis extending substantially through the center of the region of interest and perpendicular to the sectional planes, among three orthogonal axes of the three-dimensional lattice, made larger than lattice spacing in the two other directions.