Particle beam irradiation system and particle beam irradiation facility

An installation area of a particle beam treatment system is reduced. The particle beam irradiation system includes an accelerator which is installed on a floor surface and accelerates a charged particle beam, a transport device which transports the charged particle beam emitted from the accelerator, an irradiation device which irradiates an irradiation target with the charged particle beam transported by the transport device, and a gantry which is installed on the floor surface, and to which the irradiation device is attached. The gantry includes a rotating body which rotates the irradiation device about the irradiation target, and a support device which supports the rotating body from the floor surface at a position where a projection plane of the rotating body on the floor surface and a projection plane of the accelerator or the transport device on the floor surface at least partially overlap with each other.