The present invention relates to a chamfer device for machining a weld bead surface, which specifically carries out a chamfering process on the welding bead surfaces of objects to be processed (such as metal sheet materials and pipes). In the present invention, immediately after a chamfering-amount-adjustment unit has been rotated and released, a position-setting pin is inserted into a securing recess in the chamfering-amount-adjustment unit, while the chamfering amount is adjusted in precise units and simultaneously locked at a predetermined chamfering amount which is automatically maintained, and the adjustment of the chamfering amount is achieved in a straightforward fashion with just one touch. When the chamfering-amount-adjustment unit is operated, a mobile cutter shaft moves vertically and as it does so the chamfering amount is adjusted in such a way that the chamfering amount can be immediately and rapidly adjusted while continuously operating without regard to any cutter rotation action.