In the case where a silicon substance having a high theoretical capacity as a negative electrode active material for a lithium ion secondary battery is used as a negative electrode active material, such a negative electrode active material is provided that has a high initial battery capacity and suffers less deterioration in performance even when many cycles of charge and discharge are repeated. A lithium ion secondary battery using the negative electrode active material is provided. Silicon and copper (II) oxide, or silicon, metallic copper and water are pulverized and simultaneously mixed in a pulverization device, thereby providing a negative electrode active material that has good cycle characteristics and a large battery capacity.