A flat cell of the present invention has: a power-generating element which includes a positive plate, a negative plate, and a separator provided between the positive and negative plates; a sheath member which is composed of a upper sheath member and a lower sheath member and accommodates the power-generating element; and positive and negative terminals connected to the power-generating element through a plurality of collectors and extended from a outer periphery of the sheath member. The upper sheath member includes a flat wall in a portion accommodating the power-generating element and an inclined wall between the flat wall and the outer periphery of the sheath member. When the inside of the cell is evacuated, the inclined wall is curved toward the inside of the cell.