Provided are a negative electrode active material for a lithium ion secondary battery, which has sufficiently high discharge capacity at a high rate. The negative electrode active material containing silicon and silicon oxide includes primary particles having two phases of different compositions therein. One of the two phases has a higher silicon element concentration than the other phase, and is a fibrous phase forming a network structure in a cross section of the primary particle. Use of the negative electrode active material enables a sufficient increase in discharge capacity at a high rate.