PURPOSE: To suppress uneven finish by specifying the length from the immersed portion of a wire which is immersed in a plating bath to the front end of an abrasive grain layer disposed in a plating bath and the length of the abrasive grain layer when the wire is passed through the abrasive grain layer so that abrasive grains are deposited on the surface of the wire.

CONSTITUTION: A wire 1 is wound on a feed-side drum 2 and guided into a pretreatment tank 6 via guide rollers 3-5, and is fed to a plating bath 7 after a predetermined pretreatment. In the plating bath 7, the wire 1 is guided into an abrasive storage portion 10 by guide pulleys 8, 9, and abrasive grains as large as 120μm in grain diameter that fill the abrasive storage portion 10 are deposited on the surface of the wire 1. After the treatment in the plating bath 7, the wire 1 is rewound on a take-up-side drum 15. Therefore, the length from the immersed portion of the wire 1 which is immersed in the plating bath to the front end of an abrasive grain layer and the length of the abrasive grain layer are set to some integer multiples of a unit amount of transfer employed in intermittent feed, so as to reduce the rate of uneven finish.