Abstract: An electron beam instrument typified by a scanning electron microscope. The instrument does not suffer from defocus if the accelerating voltage is varied. The instrument has an objective lens, an electron gun emitting an electron beam, a control unit, an accelerating voltage-setting portion, first and second reference voltage sources. The control unit digitizes the accelerating voltage set by the accelerating voltage-setting portion and applies the digital voltage to the two reference voltage sources. The first voltage source consists of a first ROM and a first D/A converter. Similarly, the second voltage source consists of a second ROM and a second D/A converter. A signal V.sub.ref1 indicating the optimum objective lens current at a first working distance is stored in the first ROM. A signal .DELTA.V indicating the difference between the optimum objective lens current at a second working distance and the optimum objective lens current at the first working distance is stored in the second ROM.