A power storage device includes a pair of holding plates, several ribs and thin-walled portions. The ribs includes first ribs inclined to an extending direction of a first edge and an extending direction of a second edge and extending along straight lines connecting first engaging portions and second engaging portions, and second ribs extending along a facing direction in which the first edge and the second edge face each other. At least first ribs extend in different directions from each other and form an intersection where the at least two first ribs intersect with each other. At least one of the second ribs has opposite ends connected to the first ribs intersecting with each other.