To provide a method and a device for flattening a machined face by removing an unnecessary thin film from the machined face when a workpiece is a substrate in which a thin film is formed on a surface which is the machined face and parts having different machining rates exist and whose thickness of thin film is not uniform.

When a substrate 2 which has a groove part 2a on a surface which is a machined face 1a and on which a thin film 3 is formed is a workpiece 1, a polishing pad 8 having smaller polishing face than the machined face 1a is rotated by itself by pressing it against the machined face 1a to polish by moving the polishing pad 8 relatively for the machined face 1a for polishing. At this time, when the polishing pad 8 is pressed against the machined face 1a, a pressure applied on the machined face 1a is changed by polishing parts 4a, 4b to uniformize machining rates at each position and flatten the machined face 1a uniformly.