Aluminum is one of the candidate materials of the vacuum chamber and components in the ultra-large scale integration device production facilities, because of high thermal conductivity and minimizing weight. For that purpose, their surface must be smooth and passivated so as to have a low outgassing rate. An electropolishing method to obtain a mirror-finish and pit-free smooth surface has been developed by controlling the flow of the electrolytic fluid during polishing. The polished surface, which shows an average roughness of 0.03 mu m was found to be covered with an oxide film of 200 nm thick as determined by Auger electron spectroscopy, indicating a chemically stable surface. The outgassing rate measurement using a conduction-modulation method shows the value of 10(-10) Pa m(3) s(-1) m(-2), after baking treatment at 150 degrees C.