A machine-tool dynamic characteristic calculating device (18) comprises: a vibration unit (31a) elastically supported by a support member such that the vibration unit (31a) is slidable with respect to the support member to apply a vibration force to a rotary tool (16); which corresponds to a shift amount of the vibration unit (31a); a tool vibration detector (32) for detecting a vibration behavior of the rotary tool (16) vibrated by the vibration unit (31a); and a dynamic characteristic calculating unit (35) for calculating dynamic characteristics of a vibration system based on the vibration behavior of the rotary tool (16) in which the rotary tool (16) is at least a part of a vibration body.
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