PURPOSE: To economically cool hygroscopic powder by introducing powder with water content necessary after cooling into a fluidized layer, cooling a gas stream, which is separated from the fluidized layer and from which held-in particles are separated, on the way and repeatedly using the gas stream to the fluidized layer.

CONSTITUTION: The fluidized layer of the powder to be cooled is held in a cooling gas, the cooling gas stream and powdery particles held herein are separated from the surface of the fluidized layer, the held-in particles are separated from the gas stream, the hygroscopic powder is separated from the place at which the powder is introduced into the fluidized layer and the powder is made to have a desired temp. and a desired water content. At this time, powder having the water content necessary for the cooled powder is introduced into a fluidized bed cooling device 13 from a conduit tube 17. The gas stream which is separated from the surface of the fluidized layer of the device 13 and from which held-in particles are separated is recycled and is used in the device 13. The recycling gas stream is cooled at a position selected from many positions at inside and outside of the fluidized layer.