Abstract: A charged particle beam system includes a chamber having an opening in a ceiling, a table placed immediately below the opening and movable in one direction, a laser interferometer set in the chamber, including a laser oscillator placed along the moving direction of the table, a movable mirror placed on the table side opposing the oscillator, a beam splitter placed to cross the laser beam and a fixed mirror fixed, immediately above the beam splitter, on the ceiling of the chamber, an optical lens barrel having an opening communicating with the opening of the chamber, and a beam gun set on a ceiling of the optical lens barrel to irradiate a specimen placed on the table with a charged particle beam through the first and second openings. At least an upper wall portion from the opening to the fixed mirror of the chamber is made of an invar alloy.