An embodiment of the invention is a focusing magnet including a coil pair arranged on both sides of a path of a charged particle beam. The coil pair generates an effective magnetic field region in which a magnetic field is oriented in a direction (z-axis) perpendicular to a traveling direction (x-axis) of a charged particle beam. In an xy-plane, an incident charged particle beam deflected at a deflection angle ϕ with respect to the x-axis at a deflection point Q is deflected by the effective magnetic field region, and irradiates an isocenter at an irradiation angle θ with respect to the x-axis; an arbitrary point P2 on a boundary on an exit side of the effective magnetic field region is at an equal distance r1 from the isocenter; a point P1 on a boundary on an incident side of the effective magnetic field region and the point P2 are on a radius r2 and an arc of a central angle (θ+ϕ); and when a distance between the deflection point Q and the isocenter is L, a distance R between the deflection point Q and the point P1 satisfies a relational equation (4).