A high-performance separator for a fuel cell and a high-performance cell unit (single cell unit or short stack unit) for a fuel cell, each ensuring lightweight and compact fabrication necessary to enhance output density, enabling stacking without use of a gas seal member and making the insulating treatment of the outer circumference of the stack not necessary, and production methods thereof are provided.By a processing method of forming a thermoplastic resin composition highly filled with a carbonaceous material into a thin sheet form, heating the sheet to a melted state, and cold-shaping the melt at a high speed, a lightweight, compact and high-performance thin fuel cell separator having a corrugated flow path may be provided. Also, a separator for a fuel cell and a cell unit (single cell unit or short stack unit) for a fuel cell, where a stacked body of separator and MEA can be integrated by injection molding or heat welding and the outer circumferential part is covered with an insulating thermoplastic resin composition, may be provided.