A power storage device with high output is provided, in which the specific surface area is increased while keeping the easy-to-handle particle size of its active material. The power storage device includes a positive electrode including a positive electrode current collector and a positive electrode active material layer, a negative electrode including a negative electrode current collector and a negative electrode active material layer, and an electrolyte. The negative electrode active material layer includes a negative electrode active material which is a particle in which a plurality of slices of graphite is overlapped with each other with a gap therebetween. It is preferable that the grain diameter of the particle be 1 μm to 50 μm. Further, it is preferable that the electrolyte be in contact with the gap between the slices of graphite.