With regard to a radiation therapy planning device, the radiation planning fluoroscopic image shown on the screen of an image display monitor 6 is a large field fluoroscopic image P in which an in-progress fluoroscopic image PA is joined with a previously acquired fluoroscopic image PB. In addition, a large field fluoroscopic image P is acquired using an X-ray detector which does not produce any image distortions. As a result, the field of view for radiation planning fluoroscopic image is expanded by the amount of the previously acquired fluoroscopic image PB that is combined with the in-progress fluoroscopic image PA. In addition, with boundary line PL which is overlaid on the radiation planning fluoroscopic image, the boundary between fluoroscopic images PA and PB can be identified at a glance. As a result, even if in-progress fluoroscopic image PA and previously acquired fluoroscopic image PB is displayed simultaneously and contiguous to each other on the screen of the image display monitor 6, previously acquired fluoroscopic image PB does not interfere in the observation of the radiation planning fluoroscopic image. As a result, the field of view of the radiation planning fluoroscopic image in enlarged appropriately.