PURPOSE: To make a beam current to rapidly rise and lower by a method in which, after the beam current has risen, the value of the beam current is detected in a partial term within the term maintaining a constant state while making a correction signal comparing said detected value with the beam current setting value for controlling the output of a variable transformer on the input side.

CONSTITUTION: The respective beam current value is detected in the partial terms 11, 11', 11", 11Hz of the term in which the beam current gets constant value and in case said detected value (the average value, for instance, in the term 11 or the value in which the beam current detected value in the terms 11, 11'... are averaged excepting the unusual value) is different from the beam current setting value, a proper correction signal is made to generate for controlling output of the variable transformer 1 while moving the position of an output brush 26 by a motor. Thereby, the feedback loop is not influenced by the high voltage part so as to show fully quick response thus enabling the beam current to rapidly rise without raising the input power source frequency of the high voltage circuit.