There is provided a particle beam treatment system which has a simple mechanism and which is excellent in maintainability. Irradiation devices 10a and 10b respectively have a plurality of irradiation ports 102a, 102b, and 102c for irradiating a target with a particle beam in a plurality of directions. In addition, an irradiation nozzle unit 105 of the irradiation devices 10a and 10b is movable between the plurality of irradiation ports 102a, 102b, and 102c. A fixed cover structure 107 is disposed between the plurality of irradiation ports 102a, 102b, and 102c and the irradiation nozzle unit 105. In addition, the cover structure 107 has a slit 110 for allowing the irradiation nozzle unit 105 to move therethrough.