A Si--C--O composite powder is obtained by curing a reactive silane or siloxane having crosslinkable groups through heat curing or catalytic reaction into a crosslinked product and sintering the crosslinked product in an inert gas stream at a temperature of 700-1,400.degree. C. into an inorganic state. It exhibits satisfactory cycle performance when used as the negative electrode material for non-aqueous electrolyte secondary cells.