PURPOSE: To enable processing by automatically correcting deterioration of a grinding force of a polishing belt without changing a set value of the processing program.

CONSTITUTION: A force for pressing a polishing belt to a welded bead of a work by a pressurizing cylinder 22 is controlled, the rotational number of a motor 14 is variably controlled by an inverter 27, so as to adjust the rotation of the polishing belt, and a grinding force is controlled. A controller 25 composed of a microcomputer, etc., can perform the processing program for specifying the contact pressure P and the belt peripheral speed S of the polishing belt, and a storing range 29 for the specified values of the contact pressure P and the belt peripheral speed S of the polishing belt, and a storing range 30 for the N kinds of correction data tables for automatically changing the processing condition through a process of determining the rate to be multiplied to the contact pressure P and the belt peripheral speed S specified by the processing program according to the using times of the polishing belt are provided in a storage device 28.