To accurately measure deflection of a rotary shaft by a 3-point problem, adjust a pre-load based on this result and control finish roughness within a target range.

In a machine tool having a bearing unit 3 supporting a rotary shaft 1 with a pre-load type bearing 2, driven by a motor 4 and mounting a tool T at the tip, deflection of the rotary shaft 1 is detected by at least three sensors of displacement sensors As, Sb, Sc, based on a three-point problem in an arithmetic control part 10, a shaft deflection amount is accurately obtained, based on this result, a pre-load adjusting signal is fed to a pre-load adjusting unit 11 so as to generate shaft deflection of target finish accuracy.