Provided is a novel negative electrode active substance including silicon and a method for producing the same. The negative electrode active substance for a nonaqueous secondary cell includes a silicon phase and a compound oxide phase (for instance, a CaSiO3 phase), and is obtained by mixing silicon oxide (for instance, SiO) and a silicon compound (for instance, CaSi2) including silicon and at least one element selected from the group consisting of group 2 (group 2A) elements of the Periodic Table of Elements to prepare a mixed starting material and then reacting the mixed starting material. When used in an amount that is smaller than the amount of a conventional SiO2 phase, the compound oxide phase is sufficiently effective in inhibiting decomposition of electrolyte.