To overcome the problem that a gauge substrate is easily damaged, and the productivity is reduced when scale grooves are processed by a conventional mechanical scribing method if a plastic gauge as a thickness gauge for measuring a gap of a lip in a slot die and having a measurement thickness of 100 m or less is manufactured.

The thickness scales of the plastic gauge are formed by a method selected among an etching method and a high-energy beam removal method. A chemical wet etching by alkali or acid and a physical dry etching such as a sputter etching are used as the etching method. The method for removing a surface by a decomposition and a fusion by the high-energy beam such as an ultraviolet laser and an infrared laser can be used.