PURPOSE: To eliminate the error by floating charged particles and secure the accuracy of charge detection by arranging a detecting electrode so as to be surrounded by a guard electrode.

CONSTITUTION: The ion implanted from an ion beam generating device 3 is implanted to a semiconductor substrate 2 arranged in a rotary disc to charge the semiconductor substrate 2, and the charged potential thereof is detected by a detecting electrode 4. A guard electrode 6 is provided in the periphery of the detecting electrode 4 so as to surround the detecting electrode 4, and the voltage is supplied to the guard electrode 6 by a power source 7. At the time of implanting of the ion beam, even if the floating charged particles are generated, they are adsorbed by the guard electrode 6 when the polarity of the floating charged particles and the polarity of the guard electrode 6 are different, and the floating charged particles are repulsed by the guard electrode 6 when these polarities are the same, because the guard electrode 6 is charged, and the floating charged particles can not reach the detecting electrode 4. Consequently, a measure of the charge of the substrate 2 is not hindered by the charge of the floating charged particles.