Thin battery and battery-mounted device

A thin battery which comprises: a sheet-like electrode group; a non-aqueous electrolyte with which the electrode group is impregnated; and an outer casing in which the electrode group and the non-aqueous electrolyte are hermetically contained. The electrode group is provided with: a pair of first electrodes that are positioned outermost of the electrode group; a second electrode that is arranged between the pair of first electrodes; and a separator that is interposed between each first electrode and the second electrode. Each first electrode comprises a first collector sheet and a first active material layer that is adhered to one surface of the first collector sheet. The second electrode has a polarity different from that of the first electrodes, and comprises a second collector sheet and second active material layers that are adhered to both surfaces of the second collector sheet. The content (x) of the non-aqueous electrolyte per unit area of the first active material layer is larger than the content (y) of the non-aqueous electrolyte per unit area of the second active material layer.