1. A powder classifying device comprising: a plurality of powder classifiers that impart a whirling motion to powder with whirling gas streams to classify the powder into coarse powder and fine powder, a gas supply source that supplies the plurality of powder classifiers with gas for generating the whirling gas stream, a powder supplier that supplies the plurality of powder classifiers with powder having a particle size distribution, a fine powder collecting section that collects fine powder classified by each of the plurality of powder classifiers, a coarse powder collecting section that recovers coarse powder classified by each of the plurality of powder classifiers, and a controller that controls flow rates of gases supplied to the plurality of powder classifiers so that a classification point is substantially equal among the plurality of powder classifiers.