Abstract: For determination of the expiration of the life of a cutting tool on the basis of a resistance signal from sensor lines, a true signal component indicative of the abrasion of the sensor lines should be extracted from the resistance signal. In a signal processing method according to the present invention, an output of an abrasion sensor is sampled in a predetermined sampling cycle (Step S1), and then the sampled data is passed through a median filter for elimination of a noise therefrom (Step S2). Further, the noise-free data is passed through a separation filter (Step S3). By the separation filter, reduction data having a time-serially reduced in value is separated and eliminated from the noise-free data. As a result, true data indicative of a change in the resistance of the sensor is obtained. The true data is subjected to a counter function process (Step S4) thereby to determine how many sensor lines have been cut off by the abrasion.