1. A data processing unit, wherein the data processing unit processes electric digital frame data outputted from a radiation detector at a constant frame rate, wherein the radiation detector being arranged to be opposed to a radiation source with an object therebetween in an imaging space and the radiation source comprises a point-like radiation portion emitting a radiation radially therefrom, the radiation detector receiving the radiation and outputting the frame data corresponding in intensity to the received radiation, the object containing a curved portion, the curved portion being located at a curved tomographic reference plane imaginarily and three-dimensionally set in the imaging space, a pair of the radiation source and the radiation detector being rotated around the object such that the tomographic reference plane is focused, the data processing unit comprising: a first image producer producing an image of the tomographic reference plane based on the frame data; a searcher searching, based on the image of the tomographic reference plane and the frame data, for a focused position in the curved portion, the optimal position being present in each of radiated directions of the radiation emitted radially from the point-like radiation portion, each of the radiated directions passing a corresponding one of a plurality of specified positions mapped vertically and horizontally in the image of the tomographic reference plane; and a second image producer producing a focused image by combining pixels of the focused positions in the curved portion, the focused image reflecting therein an actual size and shape of the curved portion in the imaging space.