Sliding friction between a spherical pin of 8mm in diameter and flat (disk) substrates coated with vacuum-deposited thin film was measured under ultra high vacuum pressure for various materials, various rates of film supply (8~210 nm/min), various sliding velocities (1.5~67.0 mm/s). It was found that the most effective lubrication was obtained when the adhesion between $Si_3N_4$ pin and SUS440C disk was high and that between $Si_3N_4$ pin and $Si_3N_4$ disk was low. When In film was used as a lubricant between $Si_3N_4$ pin and stainless steel disk, the friction coefficient had a value as low as 0.04. In this case, the normal load W and the sliding speed V were expressed as 10N and 24 mm/s for $10^{-6}Pa$. The dependence of $mu$ on the thickness h of the Ag film, which was used as a lubricant between $Si_3N_4$ pin and SUS440C (Q) disk was expressed as $mu$=0.12 for W=10N and V=24mm/s when the film was thicker than 100nm. A brief discussion on these relations is presented from the viewpoint of the real contact area.