An object is to provide a method of assisting designing of a particle beam therapy facility in which a layout of multiple treatment rooms is well thought out, and a method of constructing the particle beam therapy facility. The method of assisting designing of a particle beam therapy facility of this invention 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 local-concave region calculation step, the concave-region calculation-result display step and treatment-room model displacement step are repeated until an operation-termination instruction is issued.
