Provided is a mass separator (100) for performing mass separation for an ion beam (IB). The mass separator (100) includes a transfer structure (30) that is a component of a yoke (13) and move at least one of an upper yoke (13a) positioned over the beam path (L), a lower yoke (13b) positioned under the beam path (L), and a side yoke (13c, 13d) positioned at a side of the beam path (L) between a normal position (P) in the traveling of the ion beam (IB) and a retracted position (Q) that does not overlap with at least a part of the normal position (P); the yoke (13) is surrounding the beam path (L) and is made of a magnetic body.