NOVELTY - The bipolar plate comprises a bipolar plate substrate, a byproduct-decomposing layer covering the bipolar plate substrate, and channels (3) for transport of a reactant gas. The channels are: arranged on a surface of the bipolar plate substrate; space defined walls of grooves of a rib-and-groove structure formed on the bipolar plate substrate as a part of their walls; or pores formed in a matrix of a porous medium layer arranged on the surface of the bipolar plate substrate. The bipolar plate has a first side to face an anode catalyst layer of a fuel cell.    USE - The bipolar plate is useful in a fuel cell stack (claimed), where the fuel cell is: polymer electrolyte fuel cells, direct methanol fuel cells and solid oxide fuel cells; and useful in power generators and portable power sources for electric appliances or electronic appliances and mobile devices.    ADVANTAGE - The bipolar plate improves energy density of the fuel cell.    DETAILED DESCRIPTION - The bipolar plate comprises a bipolar plate substrate, a byproduct-decomposing layer covering the bipolar plate substrate, and channels (3) for transport of a reactant gas. The channels are: arranged on a surface of the bipolar plate substrate; space defined walls of grooves of a rib-and-groove structure formed on the bipolar plate substrate as a part of their walls; or pores formed in a matrix of a porous medium layer arranged on the surface of the bipolar plate substrate. The bipolar plate has a first side to face an anode catalyst layer of a fuel cell and a second side to face a cathode catalyst layer of the fuel cell. The byproduct-decomposing layer is arranged on the first side and/or the second side and arranged on walls of the channels, includes a catalyst, is capable of decomposing a byproduct formed as a result of a reaction of the fuel cell, and contains a metallic element, a binder and conducting agents consisting of a carbon, an electroconductive ceramic and a metal powder. The part of the bipolar plate substrate or part of the matrix of the porous medium layer is present in contact with the anode catalyst layer or the cathode catalyst layer. The bipolar plate substrate or the matrix of the porous medium layer is a conductor. The metallic element is supported on the conducting agent. The conducting agent has a specific surface area of greater than or equal to 10 m2/g. INDEPENDENT CLAIMS are included for:    (1) a fuel cell; and    (2) a fuel cell stack.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a structure of a bipolar plate.    Flat portions (2)    Channels (3)    Manifolds (4)    Gasket (5)    Ribs. (111)