PURPOSE: To accurately measure the position of ion beam, by allowing heavy ion beam to pass through a membrane and measuring the intensity distribution of ion beam excited at that time.

CONSTITUTION: A carbon membrane 1 having a thickness of several 1μm is arranged to the region permitting heavy ion beam 9 to pass in the vacuum chamber 8 of a heavy ion accelerator. The beam 9 performs interaction along with the membrane 1 to excite a bound electron. The excited electron passes through the membrane 1 to emit light and returns to a ground state. This beam is condensed by a lens 2 to be formed into an image on a multichannel beam sensor 3. The sensor 3 measures a photon one at a time to measure the intensity distribution of beam. The beam is converted to an electric signal in the sensor 3 and the converted signal is amplified by an amplifier 5 and shaped in a waveform to be sent to a display apparatus 7 and a signal memory 6. The intensity distribution waveform of beam is obtained on the display apparatus 7 and, since this distribution of beam corresponds to that of the beam 9, the position of the beam 9 can be accurately measured.