A flat type battery comprising: an electrolytic solution; a power generating element that contains electrolyte layers and a plurality of electrodes (30, 40) layered with each of the electrolyte layers therebetween, and that expands with use in a layering direction of the electrodes (30, 40); and an exterior member forming a tightly sealed space containing a space in which the power generating element is housed, and an extra space, wherein the 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 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, and the volume adjustment portion allows 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, and the first exterior member and the second exterior member have end portions (322a, 332a) joined to each other while in a state of sandwiching the power generating element in the layering direction of the power generating element, characterized in that before use, an angle θ, which is formed by a straight line (H1, H2) connecting an end portion (E1, E2) of the surface intersecting the layering direction of the power generating element with an end portion (322a, 332a) of the first exterior member or second exterior member, and a straight line intersecting the layering direction of the power generating element, satisfies 15° ≤ θ ≤ 62°, a ratio of the volume Vl of the power generating element relative to a volume Va of the tightly sealed space before use is set to 0.800 ≤ Vl/Va ≤ 0.995, and the extra space is disposed surrounding the power generating element. an electrolytic solution; a power generating element that contains electrolyte layers and a plurality of electrodes (30, 40) layered with each of the electrolyte layers therebetween, and that expands with use in a layering direction of the electrodes (30, 40); and an exterior member forming a tightly sealed space containing a space in which the power generating element is housed, and an extra space, wherein the 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 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, and the volume adjustment portion allows 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, and the first exterior member and the second exterior member have end portions (322a, 332a) joined to each other while in a state of sandwiching the power generating element in the layering direction of the power generating element, characterized in that before use, an angle θ, which is formed by a straight line (H1, H2) connecting an end portion (E1, E2) of the surface intersecting the layering direction of the power generating element with an end portion (322a, 332a) of the first exterior member or second exterior member, and a straight line intersecting the layering direction of the power generating element, satisfies 15° ≤ θ ≤ 62°, a ratio of the volume Vl of the power generating element relative to a volume Va of the tightly sealed space before use is set to 0.800 ≤ Vl/Va ≤ 0.995, and the extra space is disposed surrounding the power generating element.