In gantry type particle beam irradiation system comprising a gantry and being configured to irradiate a particle beam, which has a small emittance in X direction and a large emittance in Y direction at an extraction position of a circular accelerator, from an irradiation nozzle installed in the gantry to an irradiation target, the irradiation nozzle has a ridge filter which is installed so as for a direction in which the emittance in the X direction is maintained to tilt to a direction which is perpendicular to a ridge of the ridge filter by a predetermined angle in the state where the gantry is a reference angle, wherein an angle of the gantry by which the particle beam is transported in the state such that the emittance in the X direction and the emittance in the Y direction at the extraction position of the circular accelerator are separated and each emittance is maintained at a position where the particle beam is incident on the irradiation nozzle is defined as the reference angle of the gantry.
