The composite structure forming method disclosed in the present invention is based on an aerosol deposition method, wherein an aerosol prepared by dispersing microparticles of a brittle material in a gas is sprayed toward a base material to form a structure made of the constituent material of said brittle material microparticles on the base material. This method is characterized by the following facts: a plurality of prepared particles that are aggregates prepared by fixing a plurality of particles that comprise said brittle material microparticles are held in a holding mechanism; said prepared particles are supplied from said holding mechanism into an aerosol forming mechanism; the supplied prepared particles are broken apart in said aerosol forming mechanism to form aerosol; said aerosol is sprayed toward the base material to form a composite structure of said structure and said base material. The concentration of microparticles in the aerosol can be better stabilized, and a stable state can be maintained over a long time.