A cell that includes a positive electrode, a negative electrode and a membrane interposed between the electrodes, and that is used in a redox flow battery, wherein the membrane includes an ion permeable portion that is permeable to hydrogen ions, at least at a center of the membrane in a plan view, planar areas of the positive electrode and the negative electrode are both 250 cm2 or more, and a planar area of the ion permeable portion is smaller than each of the planar areas of the positive electrode and the negative electrode, and, in the ion permeable portion, a planar area of a facing portion that actually faces the positive electrode and the negative electrode is 50% or more and 99.9% or less of a smaller of the planar areas of the positive electrode and the negative electrode.