1. A patient positioning device for positioning a couch supporting a patient to which a charged particle beam is irradiated from a particle beam irradiation system, said patient positioning device comprising: an X-ray emission device mounted to said particle beam irradiation system, being movable between a first position located in a path of said charged particle beam and a second position located away from the path of said charged particle beam to be out of interference with advance of said charged particle beam, and emitting an X-ray in said first position; an X-ray entry device for receiving the X-ray emitted from said X-ray emission device and outputting an output signal depending on the received X-ray; an image information generator for generating second image information regarding a portion of the patient lying across the path of said charged particle beam by using the output signal outputted from said X-ray entry device; and a processing unit for executing pattern matching between a part of first image information in a first set area including an isocenter, the first image information representing a tumor in the body of the patient and serving as a reference including the isocenter, and a part of the second image information in a second set area including a position corresponding to the path of said charged particle beam, thereby producing information used for positioning of said couch.