Floating bush journal bearings are widely used to support small-sized high-speed rotating machinery. When the journal rotational speed exceeds a stability threshold speed, the bearing shows self-excited vibration, which could be disappeared as the speed further increases and the bush rotational speed levels off in the case of low supply pressure. The peculiar phenomenon can be explained qualitatively by applying the hydrodynamic lubrication model with the axial oil film rupture being considered. However, the predictions are in quantitatively poor agreement with the measurements. In the present article, intending to obtain experimental results that would be useful for future improvement of the theoretical model, the authors experimentally observe the oil film by using a test apparatus of a cylindrical journal bearing that is equivalent to the inner oil film of a high-speed floating bush journal bearing. The width of the oil film is shown to gradually reduce with an increase in journal rotational speed due to air inflow into the bearing clearance, which results in the measured bush driving torque definitely lower than the theoretical one calculated from Petroff's formula. The ratio of the measured torque to the theoretical one is found to decrease with increasing journal rotational speed although the ratio does not simply decrease in proportion to the observed oil film width.