1. A powder-classification method comprising: a mixing step in which a powder and a liquid additive are mixed together; a drying step in which the powder mixed in the mixing step is dried; a loading step in which the powder dried in the drying step is loaded into a fluid classifier; a heating step in which a gas is heated; a supplying step in which the gas heated in the heating step is supplied to the fluid classifier; and a classifying step in which the powder is classified in the fluid classifier based on a grain size of the powder.