Disclosed herein is a partially movable radiation tomography apparatus capable of superimposing an anatomical image and a functional image without causing misalignment. The apparatus of the disclosure includes a device associated with a CT gantry having a top board, and a device associated with a PET gantry. The latter device is attachable and removable to/from the former device. This apparatus can reduce positional misalignment between the images captured by these devices. The present disclosure involves making a correction for positioning the subject images captured by the devices associated with the respective gantries with respect to each other based on a calibration image on which markers m attached to those gantries are shot. Such a configuration allows for positioning the subject images with possible misalignment between the gantries taken into account, and eventually reducing positional misalignment between the subject images captured by the devices associated with the respective gantries.
