To provide a lithium ion secondary battery capable of lightening the total weight of the battery by using an aluminum foil for a current collecting foil of a negative electrode.

A positive electrode 14 and a negative electrode 16 are arranged in a container 10, both electrodes 14, 16 are separated with a separator 12, and an electrolyte 18 is poured in the container 10 to constitute a lithium ion secondary battery. Amorphous carbon is used as a negative active material 24, and the amount ratio of a positive active materia 22 and the negative active material 24 is adjusted so that the potential of the positive electrode 14 during use is 4.2V or lower and the potential of the negative electrode is 0.1V or higher. As a result, an aluminum foil can be used as a negative current collector foil 20 and the total weight of the battery is reduced.