WHAT IS CLAIMED IS:
1. An ether cellulose derivative microparticle having:
an average particle diameter of 1 to 1,000 um;
a linseed oil absorption of 50 to 1,000 mL/100 g; and an average surface pore size of 0.05 to 5 um.
2. The ether cellulose derivative microparticle according to claim 1, the ether cellulose derivative microparticle having a pore volume calculated by a mercury intrusion method of 0.05 to 5 cm3/g.
3. The ether cellulose derivative microparticle according to either claim 1 or claim 2, the ether cellulose derivative microparticle having a sphericity of equal to or more than 80.
4. The ether cellulose derivative microparticle according to any one of claims 1 to 3, the ether cellulose derivative microparticle having a particle diameter distribution index of 1 to 3.
5. The ether cellulose derivative microparticle according to any one of claims 1 to 4, the ether cellulose derivative microparticle having a bulk density of 0.05 to 1.0 g/mL.
6. The ether cellulose derivative microparticle according to any one of claims 1 to 5, the ether cellulose derivative microparticle having a degree of crystallinity of equal to or more than 1%.
7. The ether cellulose derivative microparticle according to any one of claims 1 to 6, wherein an ether cellulose derivative constituting the ether cellulose

8. The ether cellulose derivative microparticle according to claim 7, wherein the alkyl cellulose comprises ethyl cellulose.
9. A dispersion comprising the ether cellulose derivative microparticle according to any one of claims 1 to 8.
10. A method for producing the ether cellulose derivative microparticle according to any one of claims 1 to 8, the method comprising, 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 comprising a solution phase mainly comprising the ether cellulose derivative (A) and a solution phase mainly comprising the polymer (B), the two separated phases comprising approximately same solvent:
forming an emulsion of the ether cellulose derivative (A), the polymer (B), and the alcohol solvent (C); and
bringing the emulsion into contact with a poor solvent (D) for the ether cellulose derivative (A) to precipitate the ether cellulose derivative microparticle.
11. The method for producing the ether cellulose derivative microparticle according to claim 10, wherein the ether cellulose derivative (A) comprises an ether cellulose derivative having a degree of crystallinity of equal to or more than 2%.
12. A composite microparticle comprising:
the ether cellulose derivative microparticle according to any one of claims

13. The composite microparticle according to claim 12, wherein the active ingredient comprises at least one selected from a physiologically active substance, a perfume, a sweetener, an acidulant, an antioxidant, a preservative, a disinfectant, a coloring agent, an agricultural chemical, a fertilizer, a repellent, an attractant, a fungicide, a sterilant, a germicide, an antimicrobial agent, an antibacterial agent, a preservative agent, an antiseptic agent, a deodorizer, and a lubricant.
14. The composite microparticle according to claim 12 or 13, the composite microparticle further comprising on a surface a sustained-release layer configured to suppress release of the active ingredient from the composite microparticle.
15. A preparation comprising the composite microparticle according to any one of claims 12 to