There is provided a structure with suppressed thickness and high density when battery cells of a thin film solid secondary battery are stacked. Adjacent battery cells are stacked such that negative electrodes are in contact with each other and positive electrodes are in contact with each other, and arranged such that a taking-out lead electrode smaller than a negative electrode surface or a positive electrode surface is sandwiched between two negative electrodes in contact with each other or two positive electrodes in contact with each other, and the lead electrodes sandwiched between electrodes of different layers are arranged such that there is no region where all of the lead electrodes simultaneously overlap one another as viewed in a planar arrangement. As for the shape of the lead electrode, there are a strip-shaped lead electrode and a linear lead electrode. Further, a conductive sheet forming the electrode is extended to also serve as the taking-out electrode, thereby making it possible to reduce the number of lead electrodes.