A treatment system comprising: an imaging system comprising, as imaging devices, one or more radiation sources which are configured to radiate radiation to a certain object in a plurality of different directions, and a plurality of detectors which are configured to detect the radiation radiated from the radiation sources at different positions; a first acquirer which is configured to acquire a plurality of images based on the radiation detected by the plurality of detectors; a first deriver which is configured to derive a position of the object in each of the plurality of images acquired by the first acquirer; the treatment system being characterised by further comprising: a second acquirer which is configured to acquire position information representing at least one of a position and a direction of a first imaging device included in the imaging system in a three-dimensional space in which the imaging system is disposed; a second deriver which is configured to derive at least one of a position and a direction of a second imaging device included in the imaging system in the three-dimensional space based on the position of the object in the images derived by the first deriver and the position or the direction of the first imaging device represented by the position information acquired by the second acquirer; and a calibrator which is configured to perform calibration of the imaging system based on a derivation result of the second deriver. an imaging system comprising, as imaging devices, one or more radiation sources which are configured to radiate radiation to a certain object in a plurality of different directions, and a plurality of detectors which are configured to detect the radiation radiated from the radiation sources at different positions; a first acquirer which is configured to acquire a plurality of images based on the radiation detected by the plurality of detectors; a first deriver which is configured to derive a position of the object in each of the plurality of images acquired by the first acquirer; the treatment system being characterised by further comprising: a second acquirer which is configured to acquire position information representing at least one of a position and a direction of a first imaging device included in the imaging system in a three-dimensional space in which the imaging system is disposed; a second deriver which is configured to derive at least one of a position and a direction of a second imaging device included in the imaging system in the three-dimensional space based on the position of the object in the images derived by the first deriver and the position or the direction of the first imaging device represented by the position information acquired by the second acquirer; and a calibrator which is configured to perform calibration of the imaging system based on a derivation result of the second deriver. a second acquirer which is configured to acquire position information representing at least one of a position and a direction of a first imaging device included in the imaging system in a three-dimensional space in which the imaging system is disposed; a second deriver which is configured to derive at least one of a position and a direction of a second imaging device included in the imaging system in the three-dimensional space based on the position of the object in the images derived by the first deriver and the position or the direction of the first imaging device represented by the position information acquired by the second acquirer; and a calibrator which is configured to perform calibration of the imaging system based on a derivation result of the second deriver.