1. A method for classifying powder using a fluid classifier, the method comprising: a mixing step of mixing a powder with an auxiliary agent made of an alcohol; a feeding step of feeding the powder mixed at the mixing step into the fluid classifier; a heating step of heating a gas; a supplying step of supplying the gas heated at the heating step to the fluid classifier; and a classifying step of classifying the powder dispersed as fine particles in the fluid classifier based on particle diameter, wherein: (i) the powder is dispersed as fine particles as a result of vaporization of the auxiliary agent which is mixed with the powder in the fluid classifier, (ii) the powder is classified by way of a spinning air current generated in the fluid classifier, and (iii) the fluid classifier is configured to alter a temperature and speed of the spinning air current.