A radius of a rotating grindstone is specified by a new means with a simple structure out of contact with the rotating grindstone. In a grinding machine that numerically controls a feed shaft (x-axis) of a wheel spindle stock, a device comprises a non-contact detector to detect a rotating grindstone on the wheel spindle stock reaching two different fixed positions in a feed direction of the feed shaft, and an arithmetic means to compute a radius “r” of the rotating grindstone by data detected from the non-contact detector.