A device for machining the flange part of a wheel arch, wherein a machining mechanism includes, as general-purpose drive parts, first to third cylinders, and the work bending mold for bending the flange part of the wheel arch, work receiving mold, and work guide of the machining mechanism are replaceable with those of the other types. When a dedicated mold is replaced according to the shape of a work, the entire part of the machining mechanism must not be replaced, but merely the work bending mold, work receiving mold and work guide may be replaced with those of the other types.