The present invention provides a DLC film that has good adhesiveness even in a low-temperature environment, and a DLC film-forming method capable of forming this DLC film. The present invention also provides a DLC film that has excellent initial compatibility, and a DLC film-forming method capable of forming this DLC film. In the present invention, a first opposing surface (31) that faces an inner clutch plate, of a substrate (30) of an outer clutch plate (15) is covered by a DLC film (26). Also, a treatment layer (33) is formed on a surface layer portion of the substrate (30). The treatment layer (33) is formed by applying direct-current pulse voltage to the substrate (30), and generating plasma in an atmosphere that contains argon gas and hydrogen gas.