Abstract: An ingot 9 is sliced to wafers by a wire 5 which travels around grooved rollers 1 to 3. A tension applied to the wire 5 is detected by tension sensors 21, while vertical displacement of dancer rollers 19 is detected by position sensors 20. At least one of a fluctuation gap of the tension, a fluctuating velocity of the tension, an acceleration of the fluctuation, vertical movement of the dancer rollers 19, a velocity of the vertical movement and acceleration of the vertical movement is calculated from the detection results, and compared with a threshold in a calculating circuit 23 to judge a sign predicting abnormal increase of the tension. When the predicting signal appears, a command signal is outputted from the calculating circuit 22 to a controller 23 to stop operation of a wire-sawing machine or to relax the wire 5. Since rupture of the wire 5 is prevented in bud, a wire-sawing machine is efficiently driven for slicing the ingot 9 with a high productivity.