A negative electrode active material is provided for a lithium ion secondary battery having high initial charging/discharging efficiency. The negative electrode active material containing silicon and silicon oxide has two phases with different compositions therein. One of the two phases has a lower silicon element concentration than the other phase, and is a fibrous phase forming a network structure in a cross section of primary particle of the negative electrode active material. Use of the negative electrode active material enables a sufficient increase in initial charging/discharging efficiency.