In a system where phase separation into two phases occurs when an ether cellulose derivative (A), a polymer (B) different from the ether cellulose derivative (A), and an alcohol solvent (C) are mixed together, the two phases including a solution phase mainly containing the ether cellulose derivative (A) and a solution phase mainly containing the polymer (B), the two separated phases containing approximately the same solvent, an emulsion is formed and brought into contact with a poor solvent (D) to provide an ether cellulose derivative microparticle having an average particle diameter of 1 to 1,000 μm, a linseed oil absorption of 50 to 1,000 mL/100 g, and an average surface pore size of 0.05 to 5 μm.
