To make it possible to avoid mix-ups of cell holders housed in outer cases so that cell packs can be assembled correctly. An outer case (40) is provided with a case-side rib (48) protruding from the first position on the inner surface, and the cell holder (20) is provided with a holder-side projection (28) protruding from the second position on the outer surface. The cell holder (20) is configured so that the second position, at which the holder-side projection (28) is provided, is located at a different position to that for a cell holder (20B) of a different type. The first and second positions are disposed at a position at which the case-side rib (48) does not interfere with the holder-side projection (28) when the cell holder (20) is inserted into the outer case (40), and the case-side rib (48) interferes with the holder-side projection (28) when an attempt is made to insert the cell holder (20B) of a different type into the outer case (40). The first and second positions are disposed in symmetric positions centered on a predetermined axis of symmetry.