To restrain abrasion of a guide roller and/or a group roller by coating the surface of the guide roller and/or the group roller with a material having a hardness equal to that of abrasive grain fixed to the surface of a wire with fixed abrasive grains.

The surfaces of a group roller 22 and a guide roller 20 are coated with a material having a hardness equal to that of abrasive grains of a wire 14 with fixed abrasive grains. As in the wire 14 with fixed abrasive grains, diamond abrasive grains, SiC grains or the like are used, the surface of the group roller 22 is also coated with a diamond film or a SiC film. Thus, a difference in hardness between the rollers 20, 22 and the wire 14 is eliminated so that the surface of the groove of the group roller 22 or the like is inhibited from being scraped and worn away by abrasive grains.