To provide an NC machine tool capable of checking run-out accuracy of a spindle at any time.

A displacement detecting means 22 is arranged on a base in a work area of the NC machine tool 1. A run-out accuracy diagnosing means 8 is arranged for diagnosing the run-out accuracy of the spindle by comparing a calculated run-out quantity with a predetermined allowable value by calculating the run-out quantity of the spindle from a displacement quantity detected by the displacement detecting means 22. A test tool is installed on the spindle, and this is rotated with the spindle as the center, and displacement of a test tool outer peripheral surface is detected by the displacement detecting means 22, and the run-out accuracy of the spindle is diagnosed by the run-out accuracy diagnosing means 8 on the basis of the detected displacement quantity. The run-out accuracy of the spindle can be diagnosed by simple and easy work such as moving the test tool in a detecting area of the displacement detecting means 22.