Provided is a negative electrode active material that contains silicon clathrate II and that is suitable for a negative electrode of a lithium ion secondary battery. The negative electrode active material includes a silicon material in which silicon clathrate II represented by composition formula NaxSi136 (0≤x≤10) is contained and a volume of a pore having a diameter of not greater than 100 nm is not less than 0.025 cm3/g.