An assembled cell is formed by laminating a plurality of rectangular secondary batteries in a thickness direction and interposing a spacer between the rectangular secondary batteries. The rectangular secondary battery includes an electrode group in which positive and negative electrodes are wound, and the electrode group is housed in the battery container formed in a flat box shape. The spacer includes a contact part which contacts with a width direction end region of a wide side surface of the battery container, a facing part which faces a width direction intermediate region of the wide side surface, and an inclined surface which is adjacent to both ends of the facing part in a width direction of the wide side surface. The inclined surface is inclined such that a thickness of the spacer becomes asymptotically smaller in a direction toward the width direction intermediate region from the width direction end region.