To provide a reference plate for an exposure carrier mechanism provided with a guide surface satisfying smooth sliding of a photosensitive material and a low reflection factor to light beams in a visible light region, and to provide its manufacturing method.

A method of treating the guide surface 25S of the reference plate for guiding the base material side of the photosensitive material scanned in a main scanning direction by the light beam in an exposed position and carried in an auxiliary scanning direction by the carrier mechanism, comprises a step of performing rough finish-machining by a barrel device; a step for applying alumite treatment to the rough-finished guide surface; and a step for applying polishing treatment for surface roughness adjustment to the alumite-treated guide surface using an abrasive cloth containing ultrafine particles.