1. A computed-tomography system comprising: a computed-tomography apparatus which performs scanning by radiating X-rays from at least two directions to a subject and collecting projection-image-data items; a host machine, connected to the computed-tomography apparatus, configured to instruct the computed-tomography apparatus to perform the scanning and to perform reconstruction processing based on the projection-image-data items transferred from the computed-tomography apparatus; and at least one submachine, connected to the host machine, configured to perform distribution processing for the reconstruction processing, wherein the host machine includes: an input unit configured to input a distribution condition defining a condition for the distribution processing; a distribution unit configured to, based on the distribution condition, distribute projection-image-data items opposed to each other to the submachine as a set; and a reception unit configured to receive data transmitted from the submachine, the data being subjected to the distribution processing, and wherein the submachine includes: an addition unit configured to perform addition, in a combination of the opposed projection-image-data items, for adding a pixel value of one of the projection-image data items to a pixel value of a coordinate position obtained by laterally reversing the other projection-image-data item, so as to be symmetric with respect to a rotation axis.