To provide a method for producing finely powdered sea algae, capable of forming an ideal rotating flow having a concentric circular state, without forming an eccentricity of the rotating flow since the speed of a gas flow ejected in a jet mill used in a finely pulverizing process, and capable of improving collision frequency by colliding materials to be pulverized each other three dimensionally for capable of performing the fine pulverization of the sea algae such as light weighted sea tangle, wakame sea weeds, laver, etc., in a short time, reducing running cost widely and excellent in pulverization efficiency.

This jet mill used in the finely pulverizing process is equipped with a hollow state rotary pulverizing chamber, a fine powder-discharging port arranged/installed at the rotary pulverization chamber, i-pieces of feeding nozzles arranged/installed at the peripheral wall of the rotary pulverization chamber and j-pieces of pulverization nozzles of which ejection parts are arranged/installed as inclined at the peripheral wall side of the rotary pulverization chamber [wherein, i<j, and (i), (j) are each a natural number], and the ejection parts are equipped with a slit-formed ejection port, circular-shaped ejection ports formed as ≥1 so as to position on the central axis line of the slit-formed ejection port, and a slit-formed high pressure gas flow passage communicating with the slit-formed ejection port and circular ejection ports.