Study on the progress of automatic machining systems has been centered actively onto CNC machine tools recently, to lead applied production technology such as unmanned machining to a practical level. In order to make this application more efficient, in-process methods of tool wear and breakage detection are required. In this study, which is chiefly on turning, the cutting force around the tool surface is determined by use of a cutting-resistant sensor and then changed into an electric signal and compared with standards, to enable the tool-life detection system to assess the tool life. When the cutting resistance detected by the cutting-resistant sensor has increased to the level of the standards, the measuring system can decide whether the tools should be changed on the basis of a comparative circuit. The model is shown to be very reliable for a cutting-tool-life detection system.