NOVELTY - A retaining structure (29) arranged between separators (23,34) having electroconductivity, has insertion hole (30) for inserting a fastening element and fastening the laminated fuel cell. A contact surface of retaining structure is in contact with oxidizing agent electrode of separator (23). Another contact surface formed in the opposite side of retaining structure is in contact with fuel electrode of the separator (34). An insulation processing unit is provided for insulation processing of any one of the contact surfaces.    USE - Fuel cell system for supplying power to portable electronic device such as mobile telephone and digital camera. Can also be used in electric vehicle, industrial application and domestic application.    ADVANTAGE - The insulation between each laminated cell in a fuel cell stack can be ensured, while being able to ensure heat conducting characteristics without using a cooling unit. The internal earth leakage can be suppressed and thermal stability can be achieved.    DETAILED DESCRIPTION - A fuel cell stack has a membrane electrode assembly (21) is arranged between a gas diffusion layer (22) on which sealing material (27) is provided and another gas diffusion layer (26) on which flow path formation element (28) is provided. A convex portion is formed in bipolar plate so as to fit to the fuel electrode side contact surface. The through holes (24,35) and fasting element insertion hole of the retaining structure are formed in the convex portion. The retaining structure is made of metal material containing aluminum. The insulation processing unit carries out alumite processing of the material containing aluminum. The contact surface of retaining structure opposite to the surface in which insulation processing surface performs alumite processing, is installed with electrode at the time of carrying alumite processing.    DESCRIPTION OF DRAWING(S) - The drawing shows an exploded perspective view of the fuel cell in the fuel cell system.    Membrane electrode assembly (21)    Gas diffusion layers (22,26)    Separators (23,34)    Through holes (24,35)    Sealing material (27)    Flow path formation unit (28)    Retaining structure (29)    Insertion hole (30)