An apparatus for measuring dynamic rigidity of a main shaft of a machine tool with high accuracy. The apparatus includes an unbalanced master that is attached to the main shaft and of which position of a center of gravity is offset with respect to a center of rotation of the main shaft; a tachometer that measures a rotational speed of the main shaft; and a run-out amount measuring device that is positioned so as to face the unbalanced master at a predetermined position in an axial direction and is any given position in a circumferential direction, and that measures an amount of run-out of the unbalanced master. The dynamic rigidity of the main shaft is calculated based on a relation between the rotational speed measured by the tachometer and the amount of run-out measured by the run-out amount measuring device.