The objective is to obtain a bolus, with which there can be formed an irradiation field that is accurately suited to the depth-direction shape of an irradiation subject, and a particle beam therapy system. Shape of the bolus (6) is set in such a way that, when an irradiation orbit (TB) of the particle beam (B) is defined by a first slant (α) from the beam axis (XB) with respect to a first axis (Asa) that starts from the first reference point (CPa), that is perpendicular to the beam axis (XB), and that includes the first reference point (CPa) and by a second slant (β) from the beam axis (XB) with respect to a second axis (Asb) that is perpendicular to the beam axis (XB) and the first axis (Asa) and includes the second reference point (CPb) a path length (LB) within the bolus (6) of a particle beam (B) in each of the irradiation orbits (TB) defined for combinations within a predetermined range, among combinations of the first slant (α) and the second slant (β), compensates a path length (LK) from a body surface (fK) to the to-be-irradiated portion (IS).
