Abstract: A beam position monitor capable of detecting the exact position of a beam of charged particles even if the beam has a large cross sectional area with its beam center being deviated from the center of the monitor. A plurality of electrodes 4A through 4D are disposed in a vacuum chamber 3 for inducing voltages when a beam passes through the vacuum chamber. A first voltage ratio is determined based on the voltages at the respective electrodes induced by the beam to thereby detect the beam position. An antenna 10 is disposed in the vacuum chamber 3 at a beam path for inducing voltages at the respective electrodes. A second voltage ratio is determined based on the voltages at the respective electrodes induced by the antenna whereby the first voltage ratio is corrected by using the thus determined second voltage ratio. The antenna has a cross sectional area equal to or less than that of the beam.