A magnetic field control apparatus comprising: a conductive vacuum duct through which a charged particle beam passes, the vacuum duct having internal vertical walls; and a plurality of magnetic field correcting plates (2a, 2b; 5) disposed on the vacuum duct in areas at which magnetic poles of a dipole magnet for bending the charged particle beam are disposed, wherein: at least one of the plurality of magnetic field correcting plates (2a, 2b; 5) is disposed in each of four areas defined by dividing a cross section of the vacuum duct by a symmetrical surface having each of both magnetic poles of the dipole magnet defined as a mirror image and a plane which extends perpendicularly to the symmetrical surface and is equidistant from the internal vertical walls of the vacuum duct; the magnetic field correcting plates (2a, 2b; 5) are formed of a material having an electric resistivity lower than that of the vacuum duct and arranged to control a magnetic field in the vacuum duct by superimposing a magnetic field generated by an eddy current induced in the magnetic field correcting plates (2a, 2b; 5) over a magnetic field generated by an eddy current of the vacuum duct, characterised in that at least one of the four areas of the vacuum duct includes two or more magnetic field correcting plates (2a, 2b; 5). a conductive vacuum duct through which a charged particle beam passes, the vacuum duct having internal vertical walls; and a plurality of magnetic field correcting plates (2a, 2b; 5) disposed on the vacuum duct in areas at which magnetic poles of a dipole magnet for bending the charged particle beam are disposed, wherein: at least one of the plurality of magnetic field correcting plates (2a, 2b; 5) is disposed in each of four areas defined by dividing a cross section of the vacuum duct by a symmetrical surface having each of both magnetic poles of the dipole magnet defined as a mirror image and a plane which extends perpendicularly to the symmetrical surface and is equidistant from the internal vertical walls of the vacuum duct; the magnetic field correcting plates (2a, 2b; 5) are formed of a material having an electric resistivity lower than that of the vacuum duct and arranged to control a magnetic field in the vacuum duct by superimposing a magnetic field generated by an eddy current induced in the magnetic field correcting plates (2a, 2b; 5) over a magnetic field generated by an eddy current of the vacuum duct, characterised in that at least one of the four areas of the vacuum duct includes two or more magnetic field correcting plates (2a, 2b; 5). at least one of the plurality of magnetic field correcting plates (2a, 2b; 5) is disposed in each of four areas defined by dividing a cross section of the vacuum duct by a symmetrical surface having each of both magnetic poles of the dipole magnet defined as a mirror image and a plane which extends perpendicularly to the symmetrical surface and is equidistant from the internal vertical walls of the vacuum duct; the magnetic field correcting plates (2a, 2b; 5) are formed of a material having an electric resistivity lower than that of the vacuum duct and arranged to control a magnetic field in the vacuum duct by superimposing a magnetic field generated by an eddy current induced in the magnetic field correcting plates (2a, 2b; 5) over a magnetic field generated by an eddy current of the vacuum duct, characterised in that at least one of the four areas of the vacuum duct includes two or more magnetic field correcting plates (2a, 2b; 5).