PURPOSE: To prevent an orbit of charged particles from being disarrayed by a leakage magnetic field as well as to make such a particle accelerator that is less in a loss securable by installing a superconductor at the outside of a beam duct.

CONSTITUTION: In case of a high energy particle accelerator for accelerating charged particles at high speed, a cylinder and a plate consisting of superconductive material 2 or a band, or foil are rolled at least a part of a beam duct where charged particles pass through. A superconductor 2 has a magnetic shielding effect through complete conductivity and diamagnetism. Consequently, a charged particle beam passing through the inside of the beam duct 1 rolled by this superconductive material receives no influence of a leakage magnetic field. With this constitution, since an external magnetic field is shield so that a beam orbit is not disarrayed, thus a particle loss is reducible.