A method includes a freezing process in which a solution or gel containing cellulose nanofibers is frozen to obtain a frozen component, a drying process in which the frozen component is dried in a vacuum to obtain a dry component, and a carbonizing process in which the dry component is heated and carbonized in a non-combustible atmosphere, and in the carbonizing process, the dry component is heated together with a reducing catalyst and also a material that generates a reducing gas by thermal decomposition.