A flat battery basically comprises a battery body, a plate-shaped member and an elastic member. The battery body includes a power generation unit and a case member encasing and sealing the power generation unit therein. The plate-shaped member is configured to be disposed between an outer periphery portion of the battery body and an outer periphery portion of an adjacent battery body stacked that is to be stacked on the battery body. The elastic member joins the battery body and the plate-shaped member to connect the battery body and the plate-shaped member, and covers at least part of the plate-shaped member. The plate-shaped member includes an exposed part which is exposed from part of an end surface of the elastic member which covers the plate-shaped member in a thickness direction of the battery body.