PURPOSE: To allow high-precision machining by automatically calibrating the circularity of a grinding grindstone via the fluctuation level of the pocket pressure of a truer bearing during truing.

CONSTITUTION: A truer shaft 14 is rotatably supported by a fluid bearing 17, and pressure detection means 22W24 detecting the pressure of this fluid bearing 17 in the flange direction, a means 28 judging that the fluctuation of the pressure becomes a predetermined level or less, and a feed control device 30 controlling the feed shift or a truer head 15 via the signal from this judging means 28 are provided to allow the automatic calibration of the circularity of a grinding grindstone. Accordingly, truing of the grinding grindstone to the high-precision circularity can be performed, and moreover truing is never performed more than necessary, thus work efficiency is also improved.