the inside sample direction over focus in either case when the voltage is increased a focal point moves in the direction where the focal distance of the primary electron beam 4 extends that is in the direction where the primary electron beam 4 focuses over focus in the inside sample direction therefore by increasing the voltages of the electrode 4 and the sample 12 by equal amount under a certain voltage offset vd the focal distance of the primary electron beam 4 extends as a result the focus position can be moved to the over focus side and used for focus adjustment the primary electron beam 4 is decelerated at the electrode 24 and is made incident to the objective lens for this purpose the electrode 24 is arranged in the direction closer to the cathode 1 than a lens principal surface of the objective lens 11 and within a range in which the electric field of the electrode 24 is able to reach the principal surface of the objective lens 11 of course the focus position can be moved to the under focus side by decreasing the voltages of the electrode 24 and the sample 12 by equal amount in the same polarity while the voltage offset vd is being kept at the optimum value in this case also focal movement by the increase in the voltages of the electrode 24 and the sample.