To stably maintain the viscosity of lubricant and thrust bearing oil film thickness by indirectly measuring variation in the bearing oil film thickness from an axial movement.

The thrust bearing oil film thickness is indirectly measured by detecting the axial displacement of projecting rotor by a displacement meter 12. A control valve 10 is operated according to the displacement detected by a displacement meter 12k so that the axial displacement may not move to an oil film thickness reducing direction from a position capable of keeping the required oil film thickness of thrust bearing. Accordingly, the required film thickness can be kept by controlling the axial displacement by changing the temperature of lubrication oil for changing its viscousity. In this case, it is required to determine a basic position of axial displacement in order to control the axial displacement, and at first the projecting rotor is pushed in to the thrust load direction by a jack or the like to obtain a position where the thrust bearing oil film thickness becomes substantially zero. A position moved from this position to a counter-thrust load direction by a required oil film pressure serves as the basic position.