A radiotherapy system comprising: a placing unit configured to place a subject an imaging unit configured to perform an imaging of the subject an image storage unit configured to store therein first image data obtained by performing a pre-imaging of the subject in radiotherapy planning a region storage unit configured to store therein position information of a required region included in the first image data and an image aligning unit configured to use the position information of the required region to align the first image data with second image data obtained by the imaging unit performing the imaging of the subject before radiotherapy.
