A flat type battery includes an exterior member housing an electrolytic solution and a power generating element. The power generating element contains electrodes alternating layered between electrolyte layers, and expands with use in a layering direction of the electrodes. The exterior member forms a tightly sealed space in which the power generating element is housed, and in which an extra space is formed between the exterior member and a side surface extending along the layering direction of the power generating element. The exterior member includes a volume adjustment portion allowing for an increase in the volume of the extra space by expanding in response to a pressure rise inside the tightly sealed space while the exterior member is being pressed against the surfaces intersecting the layering direction of the power generating element due to a pressure difference between the exterior and the interior.