A negative active material thin film containing silicon as a main component is formed on a collector. A composition gradient layer, in which a composition distribution of a main component element of the collector and silicon is varied smoothly, is formed in the vicinity of the interface between the collector and the negative active material thin film. The composition gradient layer contains at least one kind of third element selected from W, Mo, Cr, Co, Fe, Mn, Ni, and P, in addition to the elements contained in the collector and the elements contained in the negative active material thin film. The third element irregularizes the atomic arrangement at the interface between the collector and the negative active material thin film. Therefore, even when the negative active material absorbs/desorbs ions during charging/discharging, thereby allowing silicon particles in the negative active material to expand/contract, the strain at the interface involved in the expansion/contraction of the silicon particles is alleviated, and peeling at the interface between the negative active material thin film and the collector is suppressed. Consequently, cycle characteristics are enhanced.