The present invention provides a wire saw which cuts a workpiece using a cutting wire and is capable of adjusting wire tension with high responsiveness. The wire saw includes first and second workpiece cutting units 1A and 1B. Each of the workpiece cutting units 1A and 1B includes a pair of guide rollers 10a and 10b around which a wire W is wound to form a workpiece-cutting wire group. The wire saw further includes a tension detector 18 which detects tension in the wire W between the workpiece cutting units 1A and 1B, and a control device 50. The control device 50 changes a rotational speed of the guide rollers 10a and 10b of at least one of the workpiece cutting units based on the tension detected by the tension detector 18 so as to keep the tension within an acceptable range.