PURPOSE: To measure a beam current with high accuracy without being influenced by a noise component.

CONSTITUTION: A beam current is converted into voltage by an amplifying circuit 1, and it is converted into a digital signal by an A/D converter 2. This digital signal is instantaneous electric current value data to show an instantaneous value of the beam current. Next, the instantaneous electric current value data is integrated by an integrator 3, and electric current value integrated data is obtained. This electric current value integrated data is the one (the sum total of a quantity of electric charge flowed into for constant time) obtained by integrating the beam current with time. Noise data is subtracted from the electric current value integrated data by a subtacter 4. This noise data is the one measured on the whole noise component of an altenating current and a direct current in the same way as the beam current in a beam current nonexistent condition. Thereby, by the subtraction, the electric current value integrated data becomes an accurate one which does not contain an error due to a noise component.