A silicon oxide in the form of powder is represented by SiOx, wherein, when the silicon oxide is measured by use of an X-ray diffractometer comprising a sealed tube light source as a light source and a high speed detector as a detector, a halo is detected at 20°≦2θ≦40°, and a peak is detected at the highest quartz line position therein. The height P1 of the halo and the height P2 of the peak at the highest quartz line position satisfy P2/P1≦0.05. This silicon oxide is used as the negative electrode active material, whereby a lithium-ion secondary battery having stable initial efficiency and cycle characteristic can be obtained. The x of the SiOx is preferably 0.7