PURPOSE: To prevent a mistaken work, by deciding as to whether or not it is the specified tool with the form and dimension being measured automatically, on the tool to be installed with its being moved to a tool main shaft initially from a tool magazine.

CONSTITUTION: When a start operation is performed via an operation panel 19, a machine tool 12 starts an NC work according to the command of an NC work program. After reading the NC work program on each step and executing those, if the reading step is the case of a tool changing command, it is decided whether there is a flag on the corresponding table or not. When the flag is erected the flag is erased. Then, it is decided whether the tool is the specified tool to be used hereafter or not. Then, the competent tool is installed to a spindle in the case of there being a tool changing command. It is know whether or not the length and form of the competent tool are right from the positions of X, Y, Z axis when the competent tool and contact detection means are relatively moved and brought into contact by a subroutine program, with the contact detection means being provided on the table. A display for informing to a worker is displayed on a display 18 in the case of its being not the specified tool.