A method for manufacturing a composite in which at least one of an elemental metal or a metal compound is carried on a carrier. The method includes exposing a dispersion liquid containing a solvent and the carrier to a reduced-pressure environment of less than 80 kPa (absolute pressure) at normal temperature, preparing a raw material mixture liquid by mixing a metal-ion supplying agent which is a metal ion source of the elemental metal or the metal compound with the dispersion liquid, and mixing a reducing agent with the raw material mixture liquid and causing the elemental metal or the metal compound to be carried on a surface of the carrier.