When conditions for an electron gun mainly represented by extraction voltage V1 and accelerating voltage V0 are changed, a charged particle beam is once focused on a fixed position by means of a condenser lens and a virtual cathode position is calculated from a lens excitation of the condenser lens at that time and the mechanical positional relation of the electron gun to set an optical condition. For more accurate setting of the optical condition, a deflecting electrode device is provided at a crossover position of the condenser lens and a voltage is applied to the deflecting electrode device at a constant period so as to control the lens excitation of the condenser lens such that the amount of movement of an image is minimized on an image display unit such as CRT.