PURPOSE: To enable measurement of a beam profile as well as a beam current by arranging a non-destructive detector section sensitive to the saturation of positive or negative electric charge in a beam and a non-destructive detector section sensitive to a total number of charged particles regardless of polarity, positive or negative, thereof.

CONSTITUTION: When a beam 1 mixed with positrons and electrons passes through a current detecting section 6, for example, a wall current monitor as a non-destructive detecting section which is sensitive to the sum of positive and negative electric charges in the beam, a total current J +J of the beam 1 is obtained. When the beam passes through a electrically charged particle flux detecting section 7, for example, a very thin resin scintillator as a non-destructive detecting section which is sensitive to the sum of the electrically charged particles in the beam regardless of polarity, positive or negative, of electric charge, a total of electrically charged particle fluxes ft=f +f in the beam is obtained. By processing the outputs with an arithmetic circuit 8, the desired positron current J =(Jt+eft)/2 is obtained. When the detecting section 7 is so arranged to allow the measurement of a two-dimensional beam profile and a computation is performed between pixels corresponding to the respective outputs, a beam profile of the positron can be measured as well.