PURPOSE: To prevent in advance the danger due to the installation of a tool or the nonconformance of the NC command rotation number, by detecting the notch for discrimination provided on an installing tool during the stoppage of a main shaft and discriminating the conformance of the result thereof and NC command rotation number.

CONSTITUTION: Notches 5, 5 for discrimination are formed at the position different from that of the key grooves 4c, 4c formed on the flange 4b of a tool 4 and a detector 6 is fitted to the front face of a main shaft head 1, corresponding to the stop position of the notches 5, 5 of the tool exchange time. The presence of the discriminating notches 5, 5 is detected by the detector 6 at the time when the tool 4 is installed to a main shaft 2 and whether or not the result thereof and the NC command rotation number of the main shaft 2 given to the tool 4 are conformed is discriminated. In the case of the conformance the processing by the tool 4 is carried out and in the case of nonconformance an alarm display and informing means are actuated without rotating the main shaft 2. This discrimination is performed during the stoppage of the main shaft 2.