A wire saw adapted to cut a workpiece using a cutting wire, the wire saw comprising: a wire feeder that feeds the cutting wire; a first workpiece cutting unit including a plurality of first guide rollers (10a, 10b) and a first drive unit (11A, 11B) that rotationally drives the first guide rollers (10a, 10b), the cutting wire fed from the wire feeder being wound around each of the first guide rollers (10a, 10b) to form a workpiece-cutting wire group; a second workpiece cutting unit including a plurality of second guide rollers (10a, 10b) and a second drive unit (11A, 11B) that rotationally drives the second guide rollers (10a, 10b), the cutting wire led out from the first workpiece cutting unit being wound around each of the second guide rollers (10a, 10b) to form a workpiece-cutting wire group; a wire winder that winds the cutting wire led out from the second workpiece cutting unit; and a tension detector (17, 18, 19) that detects tension in the cutting wire between the first workpiece cutting unit and the second workpiece cutting unit; characterized by a control device that adjusts respective rotational speeds of the first guide rollers (10a, 10b) and the second guide rollers (10a, 10b) driven by the first drive unit (11A, 11B) and the second drive unit (11A, 11B) respectively, the control device changing the rotational speed of at least one of the first guide rollers (10a, 10b) and the second guide rollers (10a, 10b) so as to keep the tension detected by the tension detector (17, 18, 19) within a predetermined range, wherein the control device causes the guide rollers (10a, 10b) of the workpiece cutting units (1A, 1B) to be driven at a predetermined rotational speed, wherein the control device performs a feedback control of the rotational speed of the guide rollers (10a, 10b) based on a deviation between a tension detected value by the tension detector (17, 18, 19) and an acceptable value set in advance, and wherein the control device changes only the rotational speed of one of the first guide rollers (10a, 10b) and the second guide rollers (10a, 10b) when the deviation is smaller than a certain value while it changes the rotational speeds of both the first guide rollers (10a, 10b) and the second guide rollers (10a, 10b) when the deviation is equal to or greater than the certain value. a wire feeder that feeds the cutting wire; a first workpiece cutting unit including a plurality of first guide rollers (10a, 10b) and a first drive unit (11A, 11B) that rotationally drives the first guide rollers (10a, 10b), the cutting wire fed from the wire feeder being wound around each of the first guide rollers (10a, 10b) to form a workpiece-cutting wire group; a second workpiece cutting unit including a plurality of second guide rollers (10a, 10b) and a second drive unit (11A, 11B) that rotationally drives the second guide rollers (10a, 10b), the cutting wire led out from the first workpiece cutting unit being wound around each of the second guide rollers (10a, 10b) to form a workpiece-cutting wire group; a wire winder that winds the cutting wire led out from the second workpiece cutting unit; and a tension detector (17, 18, 19) that detects tension in the cutting wire between the first workpiece cutting unit and the second workpiece cutting unit; characterized by a control device that adjusts respective rotational speeds of the first guide rollers (10a, 10b) and the second guide rollers (10a, 10b) driven by the first drive unit (11A, 11B) and the second drive unit (11A, 11B) respectively, the control device changing the rotational speed of at least one of the first guide rollers (10a, 10b) and the second guide rollers (10a, 10b) so as to keep the tension detected by the tension detector (17, 18, 19) within a predetermined range, wherein the control device causes the guide rollers (10a, 10b) of the workpiece cutting units (1A, 1B) to be driven at a predetermined rotational speed, wherein the control device performs a feedback control of the rotational speed of the guide rollers (10a, 10b) based on a deviation between a tension detected value by the tension detector (17, 18, 19) and an acceptable value set in advance, and wherein the control device changes only the rotational speed of one of the first guide rollers (10a, 10b) and the second guide rollers (10a, 10b) when the deviation is smaller than a certain value while it changes the rotational speeds of both the first guide rollers (10a, 10b) and the second guide rollers (10a, 10b) when the deviation is equal to or greater than the certain value.