In order to clarify the lubrication mechanism of journal sliding bearing in a low rotational speed region, the relation between the friction coefficient etc and the minimum gap, which are defined between a shaft and a journal sliding bearing, has been obtained by the method of measuring the voltage between the shaft and the bearing for the minimum gap. As a result, it has been found that the friction coefficient largely decreases up to about 0.3 μm in minimum gap and almost shows constant after that with the increase of the minimum gap. Also, it has been supposed that the mixed lubrication state, where the surface asperities of the shaft and the bearing partially contact each other, continues up to about 15 μm in minimum gap because of the rotational deflection of the shaft.