The wire saw includes a pair of reel bobbins (46,47). A strand of wire (37) is unwound from one of the reel bobbins (46,47) and wound about a plurality of cutting rollers (33,34), which constitute a cutting device (31). The wire is simultaneously wound about the other reel bobbin (47,46) while rotating the cutting rollers (33,34) to cause the wire (37) to travel. A cutting feed is applied to the wire (37) to cut the workpiece (30). The wire saw includes reel motors (48,49), which rotate the associated reel bobbins (46,47), a sensor (61,62), which detects the tension fluctuation of the wire (37), a compensating device, which alters the rotating speed of the reel motors (48,49) in correspondence with the detection data of the sensor (61,62). The wire saw further includes an altering device, which alters a compensation coefficient for altering the rotating speed of the reel motors (48,49) in correspondence with the winding diameter of the reel bobbins (46,47).