An apparatus for measuring ion beam current values without disturbing the state of ionization of an ion source includes a high-voltage circuit for applying a voltage between an anode and at least one cathode of an ion source based on a voltage condition and supplying its output current to the anode; a gas flow rate adjusting mechanism for adjusting the flow rate of a gas being an ion source material for generating ions and to be admitted into the ion source; a memory in which there is stored information representing a relationship between the flow rate of the gas and the value of an extraction current flowing through an extraction electrode; and an arithmetic processor for finding the extraction current corresponding to the flow rate of the gas based on the information stored in the memory and subtracting the value of the extraction current from the value of the output current supplied to the anode by the high-voltage circuit to compute the electrical current value of the ion beam.