NOVELTY - The separator useful in single cell unit and a short stack unit for a fuel cell, comprises an electrically conducting flow path part (2) having a resin composition, and an insulating outer circumferential part (1) surrounding the electrically conducting flow path part having a resin composition. The electrically conducting resin composition comprises a carbonaceous material and a thermoplastic resin composition at a mass ratio of 1-20 with a total mass of 80-100%. The electrically conducting flow path part has a corrugated shape having a recess and a projection.    USE - The separator is useful in single cell unit and a short stack unit for a fuel cell (claimed).    ADVANTAGE - The separator is lightweight and compact, has excellent electrical conductivity, high mechanical characteristics and high-performance, and enables the construction of a fuel cell stack without using a gasket or packing.    DETAILED DESCRIPTION - The separator useful in single cell unit and a short stack unit for a fuel cell, comprises an electrically conducting flow path part (2) having a resin composition, and an insulating outer circumferential part (1) surrounding the electrically conducting flow path part having a resin composition. The electrically conducting resin composition comprises a carbonaceous material and a thermoplastic resin composition at a mass ratio of 1-20 with a total mass of 80-100%. The electrically conducting flow path part has a corrugated shape having a recess and a projection on each of front and back surfaces, a thickness of 0.05-0.5 mm, a ratio of maximum to minimum thickness of 1-3, and a density of 1.2-2.5 g/cm3. The recess constitutes a groove for the flow path, and has a curved shape in the cross section. The projection has a flat surface. The insulating thermoplastic resin composition has a volume resistivity of 1010 Omega cm or more. The electrically conducting path part and the insulating circumferential part are integrated. A terminal for measuring the cell voltage is protruded from the electrically conducting flow path part to an outside of the insulating resin outer circumferential part. In the cross section of the electrically conducting flow path part, an offset allowing flat surfaces of the projections is provided to overlap by 0.05-0.5 mm between front and back surfaces. The carbonaceous material has a carbon nanotube having a diameter of 200 nm or less. Melting point of the outer layer on a side of the front and back surfaces is lower than the melting point of the center layer. Flexural modulus of the insulating outer circumferential part is from 4/5 to 1/200 of the flexural modulus of the electrically conducting flow path part. Average linear expansion coefficient of the insulating outer circumferential part is from 1/7 to 7 of the average linear expansion coefficient of the electrically conducting flow path part. The resin compositions comprise an elastomer component (0.05-30 mass%) having a glass transition temperature of -20degrees C or less and an average particle diameter of 0.01-5 mu m, and a crystalline hydrocarbon-based polymer having a melting point of 100degrees C or more. The carbonaceous material comprises boron (0.05-5 mass%), and a vapor grown carbon fiber (0.1-50 mass%). INDEPENDENT CLAIMS are included for:    (1) a method for producing a separator for a fuel cell;    (2) a single cell unit;    (3) a short stack unit; and    (4) a method for producing a cell unit.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of the separator for a fuel cell.    Insulating outer circumferential part (1)    Electrically conducting flow path part. (2)