To enable exact reduction of a dose for an organ especially sensitive to X-rays.

A radiation beam transmitted from a tube current given radiation source (7) comes in a detection system and the detection system supplies scan data. A computerized tomography device acquires two and three dimensional scan images of an examination object (1) by an image calculation device based on the scan data. In a reconstruction method of a projection data set imperfect upon partial scanning wherein a dose is reduced in the computerized tomography device, when a z-position has one or more pitch and imperfect projection data set, data in a line other than the projection and/or a projection data set with advancing or following 360° rotation data are used in a pre-process for reconstruction of such a z-position spiral scan along the z-axis of a patient (1) on a z-direction movable table (2) is carried out by the computerized tomography device by a computerized tomography device.