PURPOSE: To prepare the titled fine resin powder which can be used without pulverization, by heating a resin syrup sprayed under reduced pressure, thereby dehydrating the produced polycondensate.

CONSTITUTION: For example, the resin syrup 1 such as urea resin etc. in the syrup tank 2 is preheated with the preheater 5, sent to the spray dehydrator 4 kept to reduced pressure by the vacuum pump 6, and sprayed through the nozzle 7 and at the same time heated with the heater 8 to effect the dehydration of the produced polycondensate. The polycondensate collected to the bottom of the dehydrator 4 is taken out through the outlet 9 such as rotary valve, etc. as resin powder 13. The heater 8 is preferably an infrared lamp, etc.