In order to prevent a reduction in workability when inserting an insulator, the insulator is to be disposed between a stacked body having a cell stack including a plurality of stacked unit cells and an end member to be disposed outward from the cell stack in stacking directions of the plurality of unit cells, and a covering to be disposed so as to separate in a perpendicular direction to the stacking directions from a side face of the stacked body parallel to the stacking directions. In a state where an end portion of the end member in the perpendicular direction is closer to the covering than an end portion of the cell stack in the perpendicular direction, and the insulator is disposed between the stacked body and the covering, the insulator includes a planar portion for covering at least a part of the side face, and a protruded portion disposed in the planar portion and protruded toward one or more unit cells near the end member among the plurality of unit cells constituting the cell stack.