A lithium ion secondary battery has an electrode group including: a winding core; and a positive electrode and a negative electrode that are wound around the winding core. This electrode group does not have an independent separator between the positive and negative electrodes. A porous film is integrally formed on both sides of at least one of the positive electrode and the negative electrode. The negative electrode and the positive electrode are connected to the winding core by a negative electrode resin film and a positive electrode resin film, respectively. The use of such resin films makes it possible to reduce the breakage of the porous films and prevent a short-circuit between the positive and negative electrodes.