A method of assisting designing of a particle beam therapy facility includes: a local-concave region calculation step of calculating a volume of a local concave region which is a concave region between two treatment-room models arranged most adjacent to each other, among multiple treatment-room models arranged in a model space corresponding to a target space for arrangement, or a projected area of the local concave region； a concave-region calculation-result display step of displaying the volume or the projected area of the local concave region calculated in the local-concave region calculation step, on a display device of a design assisting device； and a treatment-room model displacement step of displacing the treatment-room model in the model space in response to a displacement instruction for that treatment-room model, when no operation-termination instruction is issued after the concave-region calculation-result display step； wherein the above three steps are repeated until an operation-termination instruction is issued.
