A technique may produce a composite at a low temperature by a reducing agent that is easy to handle. A technique may produce a composite in which a metal simple substance or a metal oxide derived from reduced cations, or both of them are supported on a carrier. The technique may include at least: preparing a liquid phase mixture containing at least an alcohol compound as a first reducing agent, a phosphinic acid or a salt thereof as a second reducing agent, the carrier, and a source compound of one or more cations selected including Au, Ag, Cu, Pt, Rh, Ru, Re, Pd, and/or Ir; and reducing the cations in the liquid phase mixture.