A cooling plate in which flow channels are modified to reduce temperature differences between portions of the cooling plate and provide for more uniform heat distribution within a heat generator. The flow channels of the cooling plate are formed such that a central portion of the flow channels has a greater volume than the end portions near an inlet and an outlet so that the amount of cooling water that is contained in the central portion at any one time is larger than either of the end portions near the inlet and the outlet. Likelihood of thermal deformation of the cooling plate due to thermal stress is decreased, and stability of performance of the fuel cell is increased.