PROBLEM TO BE SOLVED: To provide a composite body of silicon or a silicon oxide and a carbon nanohorn, in which volume change due to insertion and desorption of lithium ions can be relieved, and a method for producing the composite body.SOLUTION: A composite body 10 of silicon or a silicon oxide and a carbon nanohorn includes: a carbon nanohorn aggregate 21 having an opening 211; and silicon or a silicon oxide 31 involved in the carbon nanohorn aggregate 21.