A plurality of single cells 10a to 10d, each being packaged by laminate films 12 and 13, are attached onto one surface of a flexible printed circuit board 20, in a state of being interconnected in series to each other, by the use of a double faced tape. Single cells 10a' to 10d' are attached in a similar way onto the other surface of the flexible printed circuit board 20. Each group of the cells on the both surfaces of the flexible printed circuit board 20 are interconnected in parallel. Voltage detection lines 30a to 30e are wired on the flexible printed circuit board 20. Positive and negative electrode tabs 14 and 15 of the respective single cells are connected to the respectively corresponding voltage detection lines 30a to 30e through connection terminals 31a to 31e provided on the flexible printed circuit board 20.