An electrode group includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a positive electrode lead electrically connected to the positive electrode. The positive electrode includes a positive electrode current collector and a positive electrode active material layer carried on each main surface of the positive electrode current collector. The positive electrode current collector has an exposed section that does not carry the positive electrode active material layer. The negative electrode includes a negative electrode current collector and a negative electrode active material layer carried on each main surface of the negative electrode current collector and has a first region and a second region. The mass of the negative electrode active material layer in the first region per unit area is smaller than the mass of the negative electrode active material layer in the second region per unit area.