An agitation method for mixing a solution and a solvent to precipitate a solid substance from the solution. The method includes preparing an agitation mixer including an agitation vessel, an impeller rotated in the agitation vessel, and a feed pipe connected to the agitation vessel and having a multiple pipe structure including an outer pipe and an inner pipe arranged in the outer pipe. A shearing clearance is formed in the agitation vessel between the impeller and the feed pipe. The method further includes shearing the solution and the solvent by rotating the impeller to precipitate the solid substance while feeding the solution and the solvent into the shearing clearance from the outer pipe and the inner pipe.