PURPOSE: To permit a grindstone to be set close to a correct position by installing, on a grindstone base, a matering tool which pivotally supports an arm equipped with a contactor inserted into a screw groove, when a long screw is subjected to continued cutting while the pitch-groove error on a screw rod is being measured.

CONSTITUTION: When screw cutting is carried-out by a grindstone by a certain length L, cutting is stopped once, and a screw rod 1 is shifted leftward. The contactor 7 of a displacement measuring tool 6 is inserted into a screw groove 1a at the edge part of the screw, and correct positioning for the screw rod 1 is performed. When the connection position of the screw rod 1 is correct, an instrument 10 indicates zero position, while when the connection position is incorrect the indicated position on the instrument is deflected by the amount corresponding to the amount of displacement. Since said contactor 7 is installed at the top edge of an arm member 9 and pivotally supported by a pin 8, any minute displacement can be magnified.