PURPOSE: To attempt improvement of operating efficiency by arranging a plurality of tool length measuring units on a table, and a programmable controller controlling the same quantity of auxiliary spindle heads, close to an NC device whereby correcting a tool working position automatically.

CONSTITUTION: Tool length measuring units 35 and 37 functioning as a tool correcting value detecting means are arranged on a table 15, and a programmable controller 39 controlling auxiliary spindle heads 17 and 19 is also arranged close to an NC device 25. And tools are housed at a predetermined position in tool magazines 11 and 13, and a work is positioned in place on the table 15. Subsequently, specified tools are taken out so as to be inserted into main spindles 21 and 23. In this case, if a tool on the auxiliary spindle head 17 is long, the same abuts on the tool length measuring unit 35 at its tip end so as to be suspended. Subsequently, a tool correction value is operated for lowering the auxiliary spindle head 19 wherein the same abuts on the tool length measuring unit 37 so as to be suspended. This constitution saves the number of workers and manhours required whereby improving operating efficiency.