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, each of the plurality of powder classifiers including a coarse powder outlet that discharges coarse 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 plurality of dumpers corresponding to the plurality of powder classifiers, each dumper being connected to the coarse powder outlet of the corresponding powder classifier to open and close the coarse powder outlet of a corresponding powder classifier, a coarse powder collecting container common to the plurality of power classifiers, the coarse powder collecting container being connected to the plurality of dumpers to recover 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, wherein the plurality of dumpers open the coarse powder outlets of the plurality of powder classifiers sequentially one at a time to discharge coarse powder into the coarse powder collecting container to prohibit circulation of gases between the plurality of powder classifiers through the plurality of dumpers and the common coarse powder collecting container.