a sample chamber having a first internal space for irradiating a focused beam onto a wafer in a vacuum condition;

a sample stage for shifting said wafer in said first internal space;

a load lock chamber having a second internal space which is changed over between a vacuum condition and an atmospherically opened condition;

a wafer exchange portion for exchanging said wafer on said sample stage with another wafer; and

a gate valve allowing said wafers to be exchanged to pass through, for communicating and isolating between said first internal chamber and said second internal chamber,

wherein said wafer exchange portion comprises:

a first arm which is slid longitudinally so as to reciprocate between said first internal space and said second internal space;

a first wafer gripping part provided at a distal end of said first arm, for gripping and releasing said wafer on said sample stage;

a second arm which is slid longitudinally so as to reciprocate between said first internal space and said second internal space; and

a second wafer gripping part provided at a distal end of said second arm, for gripping and releasing said wafer on said sample stage.