A REDOX flow battery or fuel battery includes a reactant storing and collecting unit for preventing leakage of a reactant. The battery includes two end plates and a stack that is disposed between the end plates and includes one or more unit cells. The reactant storing and collecting unit for preventing leakage of a reactant is disposed inside or outside the stack. According to the present invention, sealing reliability of the REDOX battery or fuel battery is dramatically improved. In addition, although a reactant or a product leaks from the stack, the reactant or the product may not leak to an outside of the battery but is collected before leaking to the outside. Therefore, the battery according to the present invention has an advantage of easy maintenance.