A holding member attached to outer peripheries of a plurality of adjacent cylindrical cell modules which are arranged parallel along axes thereof. Each cell module is formed by a plurality of unit cells serially connected, and the holding member is positioned in the middle of the cell modules along the axes thereof so that a gap is secured between the cell modules. The holding member has a plurality of circular holding portions for respectively holding the cell modules, wherein the holding portions are coupled with each other, and the coupled holding portions have a plurality of divided portions at which the coupled holding portions are divided into two holding bodies, each being fit over the cell modules; and a coupling device for coupling the divided portions positioned between adjacent two of the holding portions.