PURPOSE: To make it possible to automatically determine whether the position of a tool is accurate or not, by providing a magnetic scale on the tool or a tool holder so that a signal is obtained by a magnetic head on a tool attaching part.

CONSTITUTION: A magnetic scale 3 is provided to the shank 2 of a tool 1. A magnetic head 6 is provided on a tool attaching part 4 by means of a head holder 5 so that the magnetic head 6 is opposed to the magnetic scale 3, and detects magnetism on the magnetic scale and issues a signal which is fed to a computing device 9 for automatically discriminating whether the tool is attached accurately to a predetermined position in the tool attaching part or not.