Provided is a flat non-aqueous electrolyte secondary cell and a group of cells by which it is possible to achieve excellent charging/discharging cycles even if the capacitance at the cathode is increased. A flat non-aqueous electrolyte secondary cell of one aspect of the present invention is provided with: a cathode plate in which a cathode composite layer containing a cathode active material able to reversibly absorb and discharge lithium is formed; an anode plate in which an anode composite layer containing an anode active material able to reversibly absorb and discharge lithium is formed; an electrode body having a structure in which the cathode plate and the anode plate are stacked across a separator; and a non-aqueous electrolyte. In the cathode composite layer, a compound including at least one element selected from among a group of elements (M) belonging to Group 5 of the periodic table is present, and the flat non-aqueous electrolyte secondary cell has pressure applied from outside in the direction in which the cathode plate, the anode plate, and the separator are stacked.