PURPOSE: To easily and accurately judge an arrival of the anticipated life of a tap by providing a judging means which judges whether or not the position of the center of gravity is located in a range of specified coordinates, and outputs a judging signal indicating an arrival of the anticipated life of the tap when the center of gravity is out of the specified range.

CONSTITUTION: An oil-pressure electric motor is used as a tap rotation drive source, and the input side pressure and the output side pressure of the oil- pressure electric motor are detected by respective pressure sensors 81A and 81B. And these input side and output side pressure are compared with each other at the time of tapping by a comparing means 87, so that deviation between them is thereby obtained. The aforesaid deviation is differentiated with time by a deviation differentiating means 89, and the specified plural stroke portions of tapping are stored by a centripetal means 90, so that the center of gravity is thereby computed. It is then judged by a life judging means 93 whether or not the position of the center of gravity obtained by the centripetal means 90 is located in the range of the specified coordinates, and a judging signal indicating an arrival of the anticipated life of the tap is outputted when the position of the center of gravity is out of the specified range.