A novel ultrahigh vacuum (UHV) sample transfer system was constructed for the purpose of transferring Cl-2-gas etched GaAs samples from an in situ processing UHV multichamber to a scanning tunneling microscopy (STM) chamber. First, the performance of the sample transfer system was confirmed by an UHV-STM observation of an As-stabilized (2 X 4) structure of a molecular-beam epitaxially (MBE) grown GaAs (001) surface. Next, four kinds of Cl-2-gas etched samples were prepared in the UHV multichamber, and an UHV-STM observation of these etched surfaces was carried out. A strong dependence of the surface roughness on the etching temperature was observed; a 200 degrees C Cl-2-gas etched GaAs (001) surface exhibited a corrugation amplitude of several nanometers, which was about 1.5-times smaller than that of a 70 degrees C etched surface. Through annealing under an arsenic pressure at 600 degrees C which is a typical temperature for MBE growth, the smoothness of the etched surfaces remarkably improved; the corrugation amplitude of the 70 degrees C etched surface decreased by a factor of 2, and that of the 200 degrees C etched surface decreased by a factor of 3-4. As a result, the 200 degrees C etched surface has become very smooth, being only about 1.5-times rougher than the MBE grown surface. This indicates the usefulness of the in situ fabrication processes, since an interface formed by Cl-2-gas etching and following MBE regrowth has been guaranteed to be nearly as smooth as an as-grown surface.