Flat type battery

A flat type battery comprising:

• an electrolytic solution;
• a power generating element that contains electrolyte layers and a plurality of electrodes layered with each of the electrolyte layers therebetween, and that expands with use in a layering direction of the electrodes;
• and
• an exterior member forming a tightly sealed space containing a space in which the power generating element is housed, and an extra space formed between the exterior member and a side surface extending along the layering direction of the power generating element,
• the exterior member including a volume adjustment portion that allows for an increase in a volume of the extra space by expanding in accordance with a pressure rise inside the tightly sealed space while the exterior member is being pressed against surfaces intersecting the layering direction of the power generating element due to a pressure difference between the exterior and the interior,
• the volume adjustment portion allowing for an increase in the volume of the extra space while maintaining a state in which the exterior member is pressed against the surfaces intersecting the layering direction of the power generating element,
• the exterior member includes a first exterior member and a second exterior member,
• the first exterior member and the second exterior member have end portions joined to each other while in a state of sandwiching the power generating element in the layering direction of the power generating element, and
• before use an angle θ, which is formed between a straight line connecting the end portions where the first exterior member and the second exterior member are joined and an end portion of the surface intersecting the layering direction of the power generating element and a plane intersecting the layering direction of the power generating element, satisfies 15°≤θ≤34.2°.