To provide a dynamic rigidity measuring device and method of a main shaft in a machine tool, for highly accurately acquiring dynamic rigidity.

The dynamic rigidity measuring device 1 of a main shaft 6 in a machine tool 5, has: an unbalance master 2 attached to the main shaft 6 through a holder 7 and having a gravity center position eccentric from the center of rotation of the main shaft 6; a tachometer 3 measuring a rotational speed of the main shaft 6; and a swing quantity measuring instrument 4 opposed to a position in an optional circumferential direction C that is a position in a predetermined axial direction L with respect to the unbalance master 2 to measure a swing quantity of the unbalance master 2. The dynamic rigidity measuring device 1 calculates the dynamic rigidity of the main shaft 6, based on a relation between the rotational speed measured by the tachometer 3 and the swing quantity measured by the swing amount measure 4.