An object is to improve characteristics of a power storage device. The present invention relates to an electricity storage device comprising a current collector and a negative electrode-active material layer formed over the current collector. The negative electrode-active material layer includes a negative electrode comprising a first negative electrode layer in contact with the current collector; a second negative electrode layer in contact with the first negative electrode layer, having a smaller capacitance than the first negative electrode layer and containing one material selected from a nitride of lithium and a transition metal represented by LiaMbNz (M is a transition metal, 0.1≦a≦2.8, 0.2≦b≦1 and 0.6≦z≦1.4), a silicon material, and lithium titanate; a positive electrode that is paired with the negative electrode; and a solid electrolyte interposed between the positive electrode and the negative electrode.