PURPOSE: To retain the tensile force constant, upgrade the dimension accuracy of a cut face and make the life of a blade longer by sensing the displacement of the blade by means of a sensor and controlling the coolant temperature for cooling according to the sensed value.

CONSTITUTION: The sensing output of a blade displacement sensor 10 is applied to a displacement/calorie converter 13. The sensed blade displacement value is converted to calorie by a displacement-calorie converter 13, and temperature adjusting devices 11 and 12 are controlled respectively according to its result. As the loosening of tensile force of a blade 4 is relaxed in proportion to the increase of the number of cuts cut by the blade 4 by said control, the coolant temperature is lowered gradually (lowered by lowering the power of the heater), by which the tensile force can be retained constant to decrease the blade displacement. Also, the displacement of the blade 4 to a rotating shaft of the blade 4 in the vertical face direction can be performed by providing the temperature difference between the surface and back face of the blade 4 by the use of coolant pipes 8 and 9 to retain the flatness of the blade 4.