To provide a device and a method for detecting tool wear to decide a threshold through a simple method even when a kind and the cutting condition of a tool are changed and detecting precision of the tool wear is improved.

A cutting force exerted on the tool of a machine tool 1 is detected by a cutting force sensor 1b, and a sensor output therefrom is integrated with a time per one full turn of the tool 1 by an integrating device 3a is divided into a process of cutting feed and a process of feed stop cutting and stored at a memory device 3b. A computing device 3c determines an average of two integration value at each process and further a ratio of an average value of the two integration values is determined. The magnitude of an average value ratio and the magnitude of a threshold stored at a threshold memory device 3d are compared with each other by a comparing device 3e. As a result of comparison, when the average ratio is higher than a threshold, a tool wear detecting signal is outputted to a control device 1a for the machine tool 1.